The battle of rhetorical persuasion over Iran is focused in significant part on evidence. After Iran shot down a U.S. drone last month, the leadership in Tehran claimed to be acting in accordance with Article 51 of the United Nations Charter, which allows military action in self-defense. Iran proclaimed it had “indisputable” evidence that the U.S. drone was violating Iranian air space. Russia supported the Iranian claim, saying that it, too, had intelligence showing the drone to be in Iranian air space. The United States, on the other hand, declared “beyond any doubt”  that the drone was in international airspace, and blamed Iran for spreading “pure and blatant disinformation.”

Likewise earlier in the month, the United States claimed it had “lots of evidence” that Iran was responsible for the attack on two oil tankers in the Strait of Hormuz. Not everyone was convinced. Iran denied the allegations and held that they were made withouta shred of factual or circumstantial evidence.”

The question of evidence is once again at the forefront of international debate. So, how much evidence does a State need to have before it can justify using force under international law?

The Evidentiary Standard for Self-Defense

In almost every legal proceeding, there are two aspects to the question of evidence: the burden of proof and the evidentiary standard (also referred to as the standard of proof). This analysis will focus on the second element and discuss the evidentiary standard of self-defense when a State is faced with the threat of an imminent armed attack.

Currently, there is no single evidentiary standard for self-defense in international law, although one may take note of Judge Sergei Borisovitch Krylov’s remarks in his dissenting opinion (page 72) in the International Court of Justice’s Corfu Channel case:

“[O]ne cannot condemn a State on the basis of probabilities. To establish international responsibility, one must have clear and indisputable facts.”

The U.S. Evidentiary Standard

A few years ago, the U.S. State Department Legal Adviser, the U.K. Attorney General and the Australian Attorney-General all did something that is not often done, though their efforts did not draw the attention that perhaps they should have received. All three delivered public speeches articulating their own government’s understanding of the law on the use of force and their government’s legal justification for the use of force against non-State actors. What is interesting about all three speeches is that each advocated the exact same evidentiary standard for self-defense, a standard that is dangerously unclear.

In April 2016, then-U.S. State Department Legal Adviser Brian Egan gave a speech citing the following words from Sir Daniel Bethlehem‘s seminal article (emphasis added):

“The absence of specific evidence of where an attack will take place or of the precise nature of an attack does not preclude a conclusion that an armed attack is imminent for purposes of the exercise of the right of self-defense, provided that there is a reasonable and objective basis for concluding that an armed attack is imminent.”

It is unclear, however, where the evidentiary standard of ‘reasonable and objective basis’ would fall on an evidentiary spectrum.

First, what does ‘reasonable and objective’ even mean? One way to determine what it might mean is to compare it to more familiar standards commonly found in international and domestic jurisdictions. For example, it clearly does not meet the highest standard of ‘beyond a reasonable doubt,’ under which there can be no reasonable doubt as to the validity of the proposition claimed. ‘Reasonable and objective’ also does not mean that the evidence must meet the standard of ‘clear and convincing,’ which requires that the proposition claimed must be substantially more probable to be true than not. Nor does it reach the lower standard of ‘preponderance of the evidence’ (or balance of probabilities), which provides that the proposition claimed only has to be more likely to be true than not. In fact, the term ‘reasonable’ does not even mean that the evidence produced needs to be particularly ‘good.’ ‘Reasonable’ could denote a standard that is lower.

Secondly, when we read the phrase ‘reasonable and objective’ with the word ‘basis,’ the standard seems to fall even lower. The term ‘basis’ can be found in the evidentiary standard ‘reasonable basis,’ which is used by the International Criminal Court when determining whether to initiate an investigation. Furthermore, in the United States, the standard of ‘probable cause,’ which must usually be met before police make an arrest, conduct a search, or receive a warrant, is a standard often explained in terms of the comparably higher standard of reasonable suspicion with an objective basis. As can be seen from both of these examples, a ‘basis’ of evidence is used as an evidentiary standard for indicating or suggesting that a party is in the wrong, i.e. in a pre-legal determination/inquiry context. A ‘basis’ may be sufficient to accuse, but surely it is not sufficient to use as a standard to determine legal liability.

Moreover, ‘basis’ implies that a complete body of evidence is not necessary or that the standard may even be satisfied by one-sided information.

The U.K.’s Evidentiary Standard

Nine months after Egan’s speech, in January 2017, then-U.K. Attorney General Jeremy Wright gave a speech and used the exact quote as above from Bethlehem’s article, followed by:

“This statement reflects and draws upon what has been a settled position of successive British Governments.”

Although some other aspects of the principles from Bethlehem’s article may reflect the settled position of successive British governments, it is unclear to me whether this is the case for the reference to Bethlehem’s evidentiary standard. Previous references to the evidentiary standard by the U.K. in its justification on the use of self-defense have used the higher standard of ‘clear evidence.’ For example, in September 2015, former Prime Minister David Cameron told the House of Commons that there was ‘clear evidence’ against the two British nationals who were killed by airstrikes in Syria in an act of self-defense by the British Government. Furthermore, the U.K. Intelligence and Security Committee of Parliament published a report in April 2017 concluding that there was ‘clear evidence’ against the two British Nationals killed. It is possible, however, that the U.K government was declaring that it had met the higher standard of ‘clear evidence’ in these instances but was not advocating that this should be the standard.

Curiously, on November 2016, David Cameron had appeared to coin a new standard, when he told the House of Commons that self-defense against ISIL in Syria could be justified as the U.K. government had a ‘solid basis of evidence’ for this course of action.

Australia’s Evidentiary Standard

In May 2017, Australian Attorney-General George Brandis gave a speech in which he espoused the same evidentiary standard that had, by then, been articulated by the United States and the U.K.:

“I can, however, state the standard that Australia applies to such questions. Australia’s longstanding view is that there must be a reasonable and objective basis for determining that an attack is imminent. And this view can only be formed on the basis of all available evidence when the assessment is made.”

Perhaps it is not surprising that the United States, the U.K., and Australia are advocating the same evidentiary standard. The U.K. speech made reference to a meeting of the five countries’ attorneys general and to the U.S. endorsement of Bethlehem’s principle on imminence in the U.S. speech. Further, the Australian speech cited discussions by the attorneys general on the principle of imminence and explicitly declared that “[t]he Australian Government agrees with the position stated by my United Kingdom counterpart.”

A Closer Look at Bethlehem’s Evidentiary Standard

If we take a closer look at Bethlehem’s article for some guidance on the meaning of ‘reasonable and objective basis,’ the following footnote is provided:

“The `reasonable and objective basis’ formula—in paragraphs 5, 7, 8, 11, and 12—requires that the conclusion is capable of being reliably supported with a high degree of confidence on the basis of credible and all reasonably available information.”

The footnote states that the conclusion that an armed attack is imminent (which would be decided by the arguably low ‘reasonable and objective basis’ standard) must be based on “credible and all reasonably available information.” What is interesting here is the use of the word ‘information’ rather than ‘evidence.’

In practice, States seldom present the information they have relied upon when exercising self-defense. But every now and then, exceptional situations do occur and a State will find itself in front of the International Court of Justice. It is only at this point that the information relied upon must be turned into ‘evidence.’ But not all information relied upon by a State, when determining whether to exercise its right of self-defense, will be admissible as evidence or even given any weight before the Court. The use of the word ‘information’ would imply a far lower evidentiary standard than ‘evidence.’ However, some comfort may be taken from the stipulation in the footnote that States are to have “a high degree of confidence” in this information.

Concluding Remarks

So, how much evidence does a State need to have before it can justify using force under international law? Well, according to the United States, the U.K., and Australia, the answer seems to be not very much at all.

There is a growing sense that not all States agree with the interpretations of the parameters of self-defense as advanced by a few States. Self-defense is an exception to the general prohibition on the use of force in international law. The key word being ‘exception.’ The potential consequences of a State acting in self-defense can include violating a State’s sovereignty, significant loss of life, mass destruction, and the beginnings of a war. Surely, a high evidentiary standard should be imposed to limit the use of this exception.

Essentially, States are the makers and breakers of international law, but they seldom explain their legal reasoning in detail. The rare act of openness by the United States, the U.K., and Australia should be applauded and encouraged. It not only allows for the building of common understandings in international law but, more importantly, it allows for a fertile platform of debate. Such debate is vital when a government risks a dangerously wrong interpretation of something so important as the evidentiary standard for self-defense.

(Acknowledgements: I am immensely grateful to Professor Adil Haque for his helpful comments and to Sir Daniel Bethlehem for his valuable insights and advice.)

IMAGE: General Amir Ali Hajizadeh (center), Iran’s Head of the Revolutionary Guard’s aerospace division, looks at reported debris from a downed US drone on display by the Revolutionary Guard in the capital Tehran on June 21, 2019. The downing of the drone — which Washington insists was over international waters but Tehran says was within its airspace — has further increased tensions between the two countries after a series of attacks on tankers the US has blamed on Iran. (Photo by MEGHDAD MADADI/AFP/Getty Images)