(Editor’s Note: This article is part of our new symposium on the ICC and the Israel-Hamas war.)

In a recent article at Just Security, we touched upon a number of potential difficulties relating to the International Criminal Court Prosecutor’s request for arrest warrants in the Israel-Hamas war. One set of difficulties involved the application of a central principle of the Rome Statute, the principle of complementarity, which requires that the Prosecutor and the Court defer to national investigative authorities who are willing and able to investigate and prosecute. In response, Prof. Kevin Jon Heller, a special advisor to the Prosecutor, wrote on X:

The complementarity section of this post by @yuvalshany1 and @amichaic is wrong, for reasons I explained in my post @opiniojuris. If #Israel actively investigates the two suspects for substantially the same conduct at the OTP, it can challenge admissibility under Art. 19.

It doesn’t matter how many #IDF soldiers #Israel is investigating. It doesn’t matter that war makes it difficult to investigate senior leaders. It doesn’t matter civil society has brought litigation re: starvation. It doesn’t matter Israel prefers commissions to prosecutions.

It also doesn’t matter that the authors (wrongly) think #Israel was somehow cheated out of its Art. 18 challenge by having the same rule applied to it that the ICC has applied to every other similarly situated State. (Afghanistan had no trouble invoking Art. 18.)

Even this is wrong: “Formally speaking, the Prosecutor appears to be right in concluding that Israel cannot invoke… complementarity in this case.” It certainly can: by immediately opening up the necessary investigation of the two suspects and advancing the investigation.

In response, one of us replied to Prof. Heller, on X as follows:

No disagreement here. The point was that at this point in time no complementarity claim is available under the same person/conduct test. And the question is whether reasonable time was afforded to get to this point. A future investigation may of course change the analysis.

Following this short exchange on social media, we would like to elaborate our position on complementarity and to focus on what we consider to be one problematic aspect of the Prosecutor’s Applications. The latter concerns the position undertaken by the Prosecutor with regard to the applicability of Article 18 (complementarity) of the Rome Statute to the current stage of the proceedings. Contrary to what Heller suggested, we never argued that Israel would not be able to claim in the future same person/substantially the same conduct complementarity, pursuant to Article 19 of the ICC Statute, if Israel investigates or prosecutes Prime Minister Benjamin Netanyahu and Defense Secretary Yoav Gallant for starvation-related crimes. Instead, we questioned and continue to question whether the Prosecutor acted in a manner that afforded Israel a reasonable opportunity to claim complementarity before the issuance of an application for arrest warrants, in a manner that is fully consistent with the complementarity provisions of the Statute. Our line of inquiry might not only suggest that application for arrest warrants was premature; it also exposes, we believe, a flaw in the system of checks and balances created by the Statute, potentially disrupting the equilibrium between the Prosecutor and the Pre-Trial Chamber and affecting the rights and interests of both member States and non-member States.

Complementarity as part of a system of checks and balances

In response to concerns expressed in the early years of the Court, mostly from U.S. government critiques, about the problem of the so-called “unaccountable Prosecutor,” supporters of the Court (including the two authors), have often referred to the system of checks and balances available under the ICC Statute, which places appropriate limits on prosecutorial discretion. These safeguards include the need to obtain judicial authorization for key decisions taken by the Prosecutor, such as opening a proprio motu (without a State request or Security Council referral) investigation pursuant to Article 15 of the ICC Statute, issuing arrest warrants, terminating referral-based investigations and presenting charges before trial.

Also subject to this system of checks and balances is the implementation of the principle of complementarity — which is designed to protect the sovereign interests of all States to exercise their own investigatory powers with respect to events related to them and to limit the exposure of their own nationals to international judicial procedures. With respect to investigations initiated through State referrals or proprio motu action, Article 18 requires the Prosecutor, at the opening of an investigation, to notify State parties and States who would normally exercise jurisdiction over the crimes concerned, and afford them a month to inform him/her whether the State is investigating or has investigated individuals “with respect to criminal acts which may constitute crimes referred to in Article 5 and which relate to the information provided in the notification to States.” According to the ICC Appeals Chamber in the Philippines case, such a notification should be sufficiently specific “to enable the [State] to provide information in relation to its domestic investigations and prosecutions under Article 18(1) of the Statute and demonstrate the degree of mirroring” at the national level of the scope of the Prosecutor’s intended investigation (para. 107; cf. Gadaffi case, para. 83).

Invocation of complementarity by a relevant State results in deferral of the process, unless the Prosecutor obtains from the Pre-Trial Chamber authorization to continue the investigation. This intricate procedure strikes a delicate balance between the Prosecutor’s extraordinary power to start an investigation, the right of States to invoke complementarity and the authority of the Pre-Trial Chamber to control the Prosecutor and protect, where appropriate, the relevant rights of States to invoke complementarity.

It should be noted that complementarity, when invoked pursuant to Article 18, is different in its features that when invoked pursuant to Article 19 (in proceedings governed by Article 17). Under Article 19, the Court has introduced a “ same person/substantially same conduct” test, requiring a great degree of similarity between the domestic investigation and the ICC charges against specific suspects. In contrast, Article 18’s complementarity criteria tend to be more flexible. They should “sufficiently mirror the scope of the Prosecutor’s intended investigation” (Philippines case, at paras. 2 & 106),  but since they are often evaluated before specific charges have been levelled against specific suspects, they are unlikely to involve “like-for-like” assessments of proceedings (see e.g., Rastan: 2014, at pp. 457-458; for instance, they may target the “same groups or categories of individuals” investigated by the Prosecutor – see Venezuela case, at paras. 8, 110 & 247; see also Afghanistan case, at para. 46).

In any case, the State will still have to show under Article 18, “an advancing process of domestic investigations and prosecutions of the same groups or categories of individuals in relation to the relevant criminality, including the patterns and forms of criminality, within a situation” (Philippines case, at para. 106). The Prosecutor is expected to review after 6 months, and at any other time in which there is a significant change of circumstances, the willingness and ability of the State to continue in carrying out a genuine investigation, and can ask the Pre-Trial Chamber to stop the deferral and authorize the continuation of his/her investigation.

Adding new events to an existing investigation

The question of whether the Prosecutor can add to his/her investigation new charges, based on events that occurred after the investigation started, has arisen before the ICC in some specific contexts. One key context in which such a question was reviewed was when the Pre-Trial Chamber has been asked to authorize an investigation on the basis of an Article 15 request (proprio motu). In the Kenya case, the Pre-Trial Chamber refused to authorize an investigation into events that occurred after the Prosecutor submitted his request for authorization to the Pre-Trial Chamber:

Article 53(l)(a) of the Statute, by referring to “a crime [which] has been or is being committed”, makes clear that the authorization to investigate may only cover those crimes that have occurred up until the time of the filing of the Prosecutor’s Request.

This decision has been confirmed by the Pre-Trial Chamber in the Côte d’Ivoire case, although the majority recognized there an exception for “continuing crimes.” The minority judge in the Côte d’Ivoire case opted for another test developed in the Mbarushimana case, that involved a parallel question whether crimes which allegedly occurred five years after the investigation commenced exceeded the “defined parameters” of the investigation into the “situation of crisis referred to the Court as ongoing.” The test established in this latter case was whether the new events were “sufficiently linked” to the ongoing situation of crisis being investigated (para. 6).

The upshot of this jurisprudence is clear: the Prosecutor does not have a carte blanche to extend investigations beyond the “defined parameters” of Article 53 investigations and Article 15 authorizations, as this would exceed his/her legal mandate and allow him/her to operate outside the effective legal control of the Pre-Trial Chamber. In our view, the same logic should also apply to Article 18 notifications: Once the Prosecutor has opened an investigation (with or without referral) and sent a notification to States, the investigation notified has obtained certain “defined parameters,” which States seeking to avail themselves of the right to complementarity must strive to mirror. A sharp deviation from these “defining parameters” should therefore invite a new notification or an update to the original one, so as to allow the States in question to adjust their mirroring domestic investigation and claim complementarity on that basis.

With the possible exception of the Venezuela case, where the Prosecution did provide additional information shortly after the original notification was issued, the practice of the ICC Prosecutor up until now has apparently been not to issue additional notifications, with relation to new events subject to investigation, which were not mentioned in the original investigation decision that was notified to States. To the best of our knowledge, the practice has not been to date challenged before the Pre-Trial Chamber. Still, we are of the view that this practice is problematic in circumstances, where the new events being investigated significantly deviate in their features from the “defining parameters” of the original notification. In such circumstances, the failure to issue a new notification effectively deprives States of the ability – indeed the prerogative and right — to invoke Article 18 complementarity with respect to the new facts and potential charges (Cf. Burundi case, at para. 19).

The sequence of events relating to the Netanyahu and Gallant arrest warrant applications

The current investigation into the situation in Palestine is based on an initial Declaration from 2015, in which the State of Palestine authorized the Prosecutor to investigate crimes committed on its territory from June 13, 2014 onward. In 2018, Palestine, which has since joined the ICC Statute, made a referral listing specific crimes. There have been other third-State referrals since, including a referral from Nov. 17, 2023 submitted by five State parties, alluding also to additional crimes not contained in the 2018 Palestinian referral. On the basis of the 2018 referral, the former Prosecutor, Fatou Bensouda, opened an investigation in 2021 (after obtaining from the Pre-trial Chamber a ruling on certain jurisdictional issues – for a discussion, see Ambos: 2021), and appended to the announcement of investigation the results of her preliminary examination. With respect to Israel, the focus of the preliminary examination which led to the investigation was predicated on three sets of events: the 2014 hostilities in Gaza (Operation “Protective Edge”), Israel’s settlement policy in the West Bank, and the use of lethal force in the context of the 2018-2019 Great March of Return demonstrations on the Gaza border. Following her decision to initiate an investigation, the Prosecutor sent Article 18 letters of notification to Israel and the State of Palestine. Israel did not react to the letter within the one month afforded to it.

Although the request for arrest warrants announced by Karim Khan, the current ICC Prosecutor, on May 20, 2024, stems from the same Palestine Situation, it obviously relates to very different set of events from those introduced by former Prosecutor Bensouda in 2021, and relates to a different temporal framework.  He plans to charge the suspects in the case with crimes committed in the context of the current war between Israel and Hamas which started on Oct. 7, 2023. While we do not doubt that the 2024 request falls within the scope of the 2018 referral (which alluded to past, ongoing, and future crimes), we note the very different patterns of fact and forms of criminality between those specified in the investigation opened in 2021 and those specified in the 2024 announcement. The present investigation relating to Israel focuses on starvation crimes in connection with humanitarian aid policies adopted following the eruption of the current Israel-Hamas war on October 7.  This set of facts and criminal charges did not exist in 2021, and, naturally, were not part of the information provided to Israel in the 2021 notification.

The ”one shot” approach to Article 18 complementarity

In his public communications, Prosecutor Khan remains attached to the principle of complementarity as a key aspect of the ICC Statute. In fact, his office recently issued a new policy document on complementarity and cooperation, and in his May 20, 2024 announcement he made the following reference to complementarity:

I also wish to emphasise that the principle of complementarity, which is at the heart of the Rome Statute, will continue to be assessed by my Office as we take action in relation to the above-listed alleged crimes and alleged perpetrators and move forward with other lines of inquiry. Complementarity, however, requires a deferral to national authorities only when they engage in independent and impartial judicial processes that do not shield suspects and are not a sham. It requires thorough investigations at all levels addressing the policies and actions underlying these applications.

At the same time, he refrained from issuing new Article 18 letters of notification to Israel and other States, officially informing them about the significant change in the direction of the investigation; nor did he invite Israel and other States to claim Article 18 complementarity.

According to Heller, such a notification is unwarranted in this stage. In a recent blog post he opined:

Art. 18, however, is no longer relevant in the Palestine situation. Fatou Bensoudsa initiated the Palestine investigation and notified the relevant States in March 2021. The one-month period for Israel or another State to ask for deferral of the investigation based on complementarity has thus long passed.

The legal construction, which the Office of the Prosecutor appears to be pursuing in this case (and in analogous cases) is therefore as follows: Since Israel was already offered an opportunity to request a deferral on the basis of the 2021 investigation into previous armed hostilities in Gaza, the Prosecutor is not legally required to offer a new opportunity to request a deferral with regard to the extension of the investigation into the new set of armed hostilities which are now taking place in Gaza. This is so even though the original notification dealt with a pattern of facts and charges, which occurred in 2014 (Operation “Protective Edge”) and 2018-2019 (Great March of Return protests), and the current request for arrest warrants deals with a different pattern of facts and charges which occurred in the new war (2023-2024). It is also notwithstanding the fact that several States thought it important enough to submit new referrals after Oct. 7, 2023, providing thereby another indication of the substantive change in the situation from the pre-existing investigation.

This “one shot” approach to the application of Article 18 appears to us to be problematic. It is not reflective of the significance of Article 18 in the scheme of checks and balances in the ICC Statute, and is inconsistent with the expectation that the Prosecutor would strive to utilize all avenues available to him to enhance complementarity (as his avowed commitment to the principle suggests). Given the speed by which the Prosecutor moved to request arrest warrants in the present case, the effect of the circumvention of Article 18 is that Israel is unlikely to be able to invoke complementarity pursuant to Article 19 now or in the near future.

The ordinary meaning of Article 18

To recall, Article 18 requires the Prosecutor, when he/she has determined that there is a reasonable basis to commence an investigation or when a new investigation is initiated, to notify all State parties and States which would normally exercise jurisdiction over the crimes concerned. States may then notify the Prosecutor within one month that they are investigating or prosecuting the crimes “which relate to the information provided in the notification to States.” The language of Article 18 suggests that the notification by the Prosecutor is intended to provide relevant States with the possibility to claim that they are already investigating or have investigated the same fact pattern and forms of criminality that the prosecutor is investigating, or to open such an investigation within the designated one month period.

From this it follows that if the Prosecutor wishes to expand the investigation into a new pattern of facts, raising new potential charges which were not part of the investigation previously opened, he/she must consider with regard to these new facts and charges the conditions established by Article 53 (we are agnostic as to whether he should formally open a brand new investigation as was done in the Central African Republic cases, albeit on the basis of two separate referrals). In the same vein, we are of the view that he/she should also consider whether to afford a new notification to State parties under Article 18(1), informing them on the new contours of the investigation which significantly deviate from the previous investigation that was notified to them before, so that States can effectively exercise their right to ask for deferral also in relation to the new facts and charges. Any other interpretation could render Article 18 devoid of substance in circumstances like the one involving the Israeli arrest warrants, in which the notification was rendered long before the relevant events took place.

The position according to which an investigation based on new patterns of facts should require, at times, a new Article 18 notification is consistent with system of checks and balances introduced in the ICC Statute. Under Article 18, interested States are entitled to request deferral and the Prosecutor can only overcome such requests by appealing to the Pre-Trial Chamber. These actors – the States and the Chamber – thus operate as checks on the power of the Prosecutor to appropriate to himself/herself legal powers of investigation that should rest, as a matter of default under the Statute, with States, having in mind the residual nature of the Court’s jurisdiction. By attaching the current investigation into post Oct. 7, 2023 events to a notification that took place in 2021 -– the Prosecutor effectively deprived both Israel and the Pre-Trial Chamber of the right and authority to control his decision to proceed with the investigation of a new factual pattern/a new set of charges. The fact that a previous Israeli government (and other potentially interested States) have decided in 2021 that they do not wish to request deferral for one set of events (stemming from previous rounds of violence in Gaza and West Bank settlement activity) – in relation to which there may not have been at the time political will or legal capacity to investigate – does not mean that the current government is unwilling or unable to investigate at present the new charges brought forward by the Prosecutor in connection with the current war.

It should also be stressed that complementarity is not only a check on the Prosecutor’s power. It is also a central factor in conferring legitimacy upon the ICC, since many States may have joined the ICC Statute on the basis of the belief that they would be given “right of way” in cases involving the investigation of their own nationals. Other States that have supported the ICC, including non-member States, have also been persuaded in part by arguments that emphasize the complementarity safeguards in the Rome Statue. Indeed, the centrality of the principle of complementarity to the legitimacy of the ICC is arguably the reason why the Rome Statute includes several instances in which the relevant State may raise the complementarity claim (albeit with different conditions attached to different claims). Bypassing Article 18 complementarity might therefore jeopardize the legitimacy of the legal process undertaken by the Court in the eyes of certain constituencies (primarily, in this case, in Israel), since it seems as if the Prosecutor is circumventing a procedural requirement exactly because a State might exercise its right to invoke it (subject to an authorization by the Pre-Trial Chamber to reject the deferral if the domestic investigation in inadequate).

Article 18’s requirement of notification is also based on other policy considerations: First, it offers a cost-effective way to conduct investigations into alleged international crimes. The Prosecutor, by definition, has limited investigative resources at his/her disposal, and when a State investigates the same pattern of facts and categories of suspects, the Prosecutor is able to direct resources to other issues, not investigated by States.  Second, and related, the Prosecutor’s narrow approach to Article 18 means that in many cases in which he/she will extend the investigation significantly beyond the initial Article 18 notification, the State would be officially notified on the concrete charges sought only when arrest warrants are requested or issued (or indeed even later for warrants held in secret) – that is, after the Prosecutor has already come to a conclusion that there are reasonable grounds that specific individuals have committed a crime (art. 58(1)). This means that the Prosecutor may have conducted a full investigation which was redundant, because a State was already investigating or preparing to investigate the same issue, and will now request complementarity under Article 19. A new duty of official notification pursuant to Article 18 would generate more certainty for the Prosecutor and for the Court that the relevant State is not investigating the same fact pattern/charges, and would reduce unnecessary overlaps in investigations.

Third, the use of Article 18 notification supports the goal of empowering national legal systems. By contrast, Article 19 complementarity creates a competition between the Prosecutor, who has already formed an opinion on the charges in question and is acting upon them, and national legal systems that might try to move fast in order to block him/her. We believe that when compared to Article 19 procedures, Article 18 notifications and requests for deferral, like applications of complementarity at the preliminary examination stage, can better foster cooperation between the ICC and national legal systems, and a proper division of work between them (pursuant to the two track approach of partnership and vigilance elaborated in the recent ICC OTP policy paper).

As a result, a better approach than the “one shot” system to Article 18 notifications might be to extend mutatis mutandis the jurisprudence developed by the Court with regard to authorization of an investigation under Article 15 to notification of an investigation under Article 18. Once an investigation has been made subject to a request for authorization or notification, a significant change in the underlying fact pattern, which implies absence of control by the Pre-Trial Chamber or a qualitatively new question of mirroring by relevant States, would simply require a new request or a new notification. Such an approach also sits well with the jurisprudence of the Court on the need to provide a sufficiently specific notification to allow for mirroring of the investigation by the State concerned.

Differences between Article 19 and Article 18 complementarity

It is important to emphasize in this regard that the solution proposed by Heller in his tweet – invocation of Article 19 complementarity – is not an adequate fix to the gap in the system of checks and balances we identify here. First, by requesting the issuance of arrest warrants, the Prosecutor has already acted in a case that prima facie belongs, according to the scheme of ICC Statute, under the jurisdiction of those States willing and able to investigate it, thus upsetting the balance which the complementarity principle was supposed to maintain. Second, when the suspects are high ranking officials, the harm caused to State interests by a premature request for an arrest warrant is significant, given that an international warrant would seriously hamper the government’s ability to engage in foreign relations. As a result, the two options of Article 18 and Article 19 complementarity are not equivalent from the point of view of the State of nationality of the suspects, and one approach harms its sovereign interests more than the other.

Third, Article 18 complementarity affords States, as detailed above, a greater degree of latitude in reviewing the pattern of facts and in identifying specific crimes and criminal suspects than the more rigid Article 19 “same persons/substantially same conduct” standard. Article 18 might also offer a more realistic timeline to deal with complicated investigations (which notably should start an “advancing process” within one month, but could take some time to home in on the responsibility of specific suspects for specific crimes).  By contrast, in order to meet Article 19 complementarity standards, the investigation must already focus on specific suspects and crimes.

In any event, we believe that the Prosecutor should have been more patient, as a matter of policy, regarding his request for arrest warrants of the Israeli leaders – if he is genuinely interested in allowing for complementarity to operate and in promoting the exercise of accountability mechanisms in Israel, which can address the responsibility of the most senior State officials (which is something we would very much to see ourselves, given the gravity of the situation in Gaza and the many allegations that Israel engaged during the war in serious violations of international criminal law). As a matter of prosecutorial policy, the Prosecutor should allow room for a State that may be willing and able to investigate, but which is in the midst of a difficult war (which it did not initiate), time to develop modalities to advance an investigation (e.g., the establishment of a commission of inquiry), in a form that would eventually lead, if the findings warrant, to criminally investigating the political and military leadership. Expecting a State to open a criminal investigation of its leadership for war-related policy decisions while the war is still raging, without Israeli law enforcement bodies themselves undertaking some sort of preliminary examination, might be asking too much. Indeed, we are not aware of any State in history that has criminally investigated the war-related conduct of its leaders in real time. Whereas the Prosecutor’s exhortations to Israel in the months preceding his May 20 announcements to comply with obligations to facilitate humanitarian aid to Gaza did not, and perhaps could not, generate yet an investigation into the criminal responsibility of the country’s senior leadership, we believe that an official resort to Article 18 could have created the kind of political and legal dynamics required to advance a domestic investigation that could culminate in that result.

Ultimately, the perception in Israel that the Prosecutor rushed to request arrest warrants makes it less likely that the charges would serve as the basis of a criminal investigation in Israel. Indeed, the perception of refusal to give the Israeli legal system a reasonable chance to operate has already led to the Court being criticized by some of its long-time supporters and by Secretary of State Anthony Blinken, suggesting that the current approach might harm the Court in the eyes of certain observers and third States.  Article 18 notification, we believe, was introduced exactly in order to avoid such unnecessary confrontations, which do not contribute to the effectiveness of the Court in advancing its mission of ending impunity.


Article 18 to the ICC Statute requires the Prosecutor to notify the relevant States regarding his/her decision to open an investigation, and provide those States the opportunity to request deferral of the investigation, because of the existence of a domestic investigation. The Prosecutor’s approach of not issuing new notifications when significantly new fact patterns giving rise to new charges emerge, dilutes the Article 18 complementarity regime and weakens the ability of the Pre-Trial Chamber to monitor the implementation of the complementarity principle. Whereas in some past cases this approach did not present a serious problem, because, for example, the new charges were closely related to the “defining parameters” of the notified investigation or no State that was willing and able to investigate existed, this is not necessarily the case with Israel. The Israeli government may have the will and ability to conduct independent investigations into the fact patterns and charges identified by the Prosecutor, that is, within realistic timeframes. Indeed, given how far off and remote is the prospect of any prosecutions of Netanyahu and Gallant in The Hague, the timeframe we have in mind may be a far faster road toward legal accountability. In this situation and in general, it would be desirable for the Prosecutor and the judges on the Court to revisit the current practice with respect to  Article 18  complementarity and to harmonize it with the Court’s approach to Article 15 authorizations and with jurisprudence on the specific content that should be included in Article 18 notifications. The interests not only of States, but of the Court’s own legitimacy, depend on getting this right.

IMAGE: International Criminal Court Prosecutor Karim Khan poses during an interview with AFP at the Cour d’Honneur of the Palais Royal in Paris on February 7, 2024. (Photo by DIMITAR DILKOFF/AFP via Getty Images)