In a recent Just Security post, Adil Ahmad Haque asks what legal rules constrain the use of military force by an intervening state acting without the host state’s consent. Recognizing the importance of determining what legal regime governs a given situation, many scholars and the recent ICRC Commentary have focused on assessing the status of non-consensual interventions. But the corollary question has gone largely ignored: what legal rules constrain the use of military force by an intervening state acting with the host state’s consent?

Consider Russia’s targeting of anti-government opposition groups in Syria or the United States’ so-called “goodwill” strikes against Pakistani-selected targets. Certainly, international law requires that states respect each other’s sovereignty and therefore that the intervening state must operate within the bounds of the host state’s consent. But are there international legal limits on what the host state may consent to? And are there any objective international legal restraints on what actions an intervening state may take pursuant to that consent?

In our forthcoming article, Consent is Not Enough, we and our co-authors argue that consent is not a blank check. Rather, both the consent-giving and consent-receiving states have a duty to independently determine whether the consent violates the host state’s obligations under international law. If, per the Tadic test, a non-international armed conflict (NIAC) exists, then international humanitarian law applies. If not, the intervening state’s actions are governed by human rights law—both its own human rights obligations, to the extent they apply extraterritorially, and the host state’s human rights obligations, which it should not be able to evade by consenting to actions it could not lawfully take itself.

Consent allows an outside state to lawfully intervene in a host state’s sovereign territory, addressing the jus ad bellum question—but it does not establish whether a NIAC exists and whether either state’s actions are governed by jus in bello. Nonetheless, consent-based interventions are often treated as presumptively lawful, due to an unfortunate conflation of different legal regimes. First, under international law, states may generally presume that another state’s consent is lawfully given. Second, the extraterritorial application of human rights law remains contested, and most states act as if they are less constrained outside their borders. As Ashley Deeks has observed, these two principles combine to produce an unreasonable result: a host state might consent to a human rights violation, and an intervening state might rely on that consent and its presumed lack of extraterritorial human rights obligations to carry it out.

This is hardly a hypothetical concern. States are increasingly requesting or consenting to outside states using force on their territory against organized non-state actors—with devastating impact on civilians. In 2011, approximately 2,000 troops from Saudi Arabia and the United Arab Emirates entered Bahrain at the invitation of the Bahraini government to assist in quelling protests. At least thirty civilians died in the civil violence, several due to “the use of excessive and unnecessary lethal force.” Or consider the subset of US targeted killings some derisively call “side payment” or “goodwill” strikes. According to these commentators, the United States sometimes agrees to target non-state actors at war with a host state—but not necessarily with the United States—in exchange for permission to enter the host state’s airspace and conduct other strikes against US-selected targets. More recently, pursuant to President Bashar al-Assad’s request, Russia has been conducting airstrikes against “all terrorists” in Syria. However, many allege that Russia is actually targeting anti-government opposition groups, resulting in the deaths of over 2000 civilians.

Our article argues that states involved in these and other consent-based interventions have an obligation to independently determine if human rights law or international humanitarian law is the governing legal regime—and act accordingly. If it consents to an unlawful action by the intervening state, the host state violates international law; if that violation is obvious, the intervening state loses its ability to presume the consent is legitimate.

Our reasoning is grounded in Article 46 of the Vienna Convention on the Law of Treaties and Articles 16 and 41 of the Draft Articles on State Responsibility. Article 46 of the VCLT provides that consent cannot be presumed valid if a “violation was manifest”—in other words, “objectively evident to any State conducting itself in the matter in accordance with normal practice and in good faith”—and concerns a “law of fundamental importance.” Presumably, shared human rights obligations will satisfy both of these requirements. Meanwhile, Articles 16 and 41 emphasize the unlawfulness of a state knowingly facilitating another state’s human rights violations. Taken together, these principles prevent an intervening state from using the legal fiction of valid consent to assist a consent-giving host state in actions that would be manifest violations of its human rights obligations. (Admittedly, this is a lesser standard than Deeks’s proposed duty to inquire.)

Some might think that requiring intervening states to act pursuant to international human rights law, rather than the presumptively more lenient international humanitarian law of NIACs, hamstrings states in responding to threats posed by armed non-state actors. Not so. International humanitarian law does not grant states carte blanche, and international human rights law does not constitute handcuffs. Both regimes allow states to respond to threats; they differ primarily in their requirements for how a state can respond. For example, international human rights law requires law enforcement officials to use the least amount of force necessary to resolve the situation; international humanitarian law does not.

International humanitarian law and international human rights law both share the common aims of protecting human life, health, and dignity. It would be deeply damaging to both legal regimes if states could interpret the interaction of the two bodies of law to create a legal vacuum in which they could cooperate to endanger individuals with impunity. Host states must not consent to actions that they could not lawfully take themselves, and intervening states may not blindly rely on host state consent as a justification for their actions. These requirements protect both the deepest commitments of the international legal order and the lives of individuals who might otherwise be caught in the crossfire.