Last night, the United States, United Kingdom, and France launched a coordinated attack in Syria, reportedly aimed at sites related to Syria’s chemical weapons program. President Trump stated that he “ordered the United States armed forces to launch precision strikes on targets associated with the chemical weapons capabilities of Syrian dictator Bashar al-Assad.” The president emphasized that “Assad launched a savage chemical weapons attack against his own innocent people,” noted that “[l]ast Saturday the Assad regime again deployed chemical weapons to slaughter innocent civilians near the town of Douma near the Syrian capital of Damascus, and stated that “[t]he purpose of our actions tonight is to establish a strong deterrent against the production, spread, and use of chemical weapons” (emphasis added).

As we wrote before Trump’s announcement, there is no apparent domestic or international legal authority for the strikes. Although the United States has not offered a formal legal argument, a number of legal theories have been floated in recent days. We address them each briefly here.

Domestic Law

The 2001 Authorization for Use of Military Force (AUMF) authorizes the strikes. Speaker of the House Paul Ryan said on Thursday that “the existing AUMF” authorizes Trump to launch strikes against Syria in response to the use of chemical weapons there. If (as most seem to think) Ryan was referring to the 2001 AUMF enacted just after 9/11, this is an absurd position. That statute authorizes the President to use force against “nations, organizations, or persons” with a link to the 9/11 attacks, i.e., al Qaeda and the Taliban. Since 9/11, presidents and courts have agreed that the AUMF extends to “associated forces,” and the Obama and Trump administrations have controversially stretched it further to include the Islamic State. But even if one acknowledges the legitimacy of these interpretive extensions, and even if these extensions would authorize the use of force in Syria against al-Qaeda or the Islamic State, the statute cannot plausibly be construed to authorize force in Syria against Syrian forces in response to the chemical weapons attacks. The Syrian government was not involved in the 9/11 attacks and it is not an associated force of any entity covered by the AUMF.

The 2002 AUMF authorizes the strikes. It is conceivable that Ryan was referring to the 2002 Iraq AUMF, and others have hinted at this statute as a possible basis for the Syria strikes. But this argument doesn’t work either. The 2002 AUMF authorizes the president “to use the Armed Forces of the United States as he determines to be necessary and appropriate in order to …defend the national security of the United States against the continuing threat posed by Iraq.” One of us (Goldsmith) once argued that this statute might “arguably” be a basis for a use of force in Syria against the Islamic State “to the extent that the ‘continuing threat posed by Iraq’ is today constituted by ISIS forces operating in tandem (and under a single military structure) in Iraq and Syria.” But even if one accepts this argument (which not everyone does), it is not relevant to yesterday’s strikes in Syria. Trump made clear that the strike was in retaliation against Assad for the use of chemical weapons against his own people in Syria. There was no claim, and there could not be one, that the strike had anything at all to do with Iraq. It is thus inconceivable that the strikes were “necessary and appropriate” to “defend the national security of the United States against the continuing threat posed by Iraq.”

The administration’s position. The Trump administration does not believe that either the 2001 or 2002 AUMF is the domestic legal authorization for these strikes. Although it has offered no official legal argument for the strikes, Secretary of Defense James Mattis made crystal clear that he believed Article II alone was the basis:

As our commander in chief, the president has the authority under Article II of the Constitution to use military force overseas to defend important U.S. national interests. The United States has an important national interest in averting a worsening catastrophe in Syria, and specifically deterring the use and proliferation of chemical weapons.

This is the very constitutional rationale that, as we explained in our prior post, permits the president “to use air power unilaterally basically whenever he sees fit.”

International Law

The strike is necessary to enforce the prohibition on the use of chemical weapons. Each of the leaders of the three states that carried out the strikes last night—Trump, U.K. Prime Minister Theresa May, and President Emmanuel Macron of France—made some version of this argument in their statements to the public announcing the strikes. The problem with this argument is that the 1997 Chemical Weapons Convention, which is the central international legal instrument on the topic, provides an enforcement system that the three powers involved in last night’s airstrikes entirely bypassed. The Convention provides, first, for investigation by the experts from the Organization for the Prohibition of Chemical Weapons—who were due to begin their investigation today (the OPCW has announced the deployment will proceed, though it is not clear if it will be delayed). Then, in situations of “particular gravity,” the Conference of the States Parties may bring a matter to the attention of the U.N. General Assembly and Security Council. Nowhere does the Convention provide for unilateral uses of force in response to a breach of the Convention. And if it had, there’s a good chance no state would ever have joined it. And putting the treaty to one side, there is no reason at all to think that any related “norm” prohibiting the use of chemical weapons can be enforced with force outside the Charter framework.

This is a humanitarian intervention. This is the argument offered in the only legal opinion issued by a government thus far—by the U.K. As we noted in our last post, there is no exception to the U.N. Charter regime for humanitarian intervention. Former Secretary General Kofi Annan, one of the best known early advocates for the principle often referenced as a basis for humanitarian intervention, the “Responsibility to Protect,” made clear that the principle required action through the U.N., not around it: “Building on our evolved understanding of sovereignty, R2P asserts that when states cannot or will not protect their populations from the worst crimes, other states, acting through the U.N., should do so.” A very small number of states have endorsed the principle that humanitarian intervention can be taken unilaterally. But that remains a minority opinion.

Three members of the Security Council think it’s legal, so it must be legal. This argument is in clear contravention of the text of the Charter. As we explained previously, the Charter strictly prohibits the use of force subject only to exceptions for consent, self-defense, or Security Council authorization, none of which are implicated here. The fact that three of the permanent five members believe the strike is lawful cannot change the meaning of the Charter. If this argument were accepted, it would both kill the prohibition on the use of force and eliminate the ability of the “Permanent Five” members of the Security Council to veto its use.

That veto, of course, is a source of immense frustration. But it was agreed to by the authors of the U.N. Charter because, as one of us (Hathaway) has explained, the Soviets would not have joined otherwise. Moreover, the primary goal of the Charter’s authors was to avoid a war between the great powers. They believed that preventing uses of force opposed by any of Permanent Five would prevent those five militarily powerful nations from ending up on opposite sides of a war. Given the rising rhetoric between Russia and the U.S. in recent days, that worry is quite relevant here. Moreover, any argument that minority approval in the Permanent Five justifies a use of force would permit Russia and China to agree to all sorts of interventionist mischief along their borders and elsewhere.

The strike is illegal but legitimate. French Foreign Minister Jean-Yves Le Drian claimed that the air strikes were “legitimate” without purporting to defend them as lawful. The “illegal but legitimate” defense has been tossed around since Kosovo. It is often presented as a legal argument, but it is not. It is a claim that illegal behavior can nonetheless, in some circumstances, be legitimate. But legitimacy is in the eye of the beholder. If “illegal but legitimate” becomes an accepted principle, then the Charter’s limits become meaningless. Nations that do not share Western conceptions of legitimacy could justify uses of force based on their own conceptions of legitimacy. In short, “illegal but legitimate” implies no legal limits on the use of force.

Editor’s note: This piece was crossposted at Lawfare.

U.S. Department of Defense photo by Army Sgt. Amber I. Smith