The United States has just launched a missile attack against Syrian air bases, apparently in response to the Assad regime’s use of chemical weapons against Syrian civilians. (The attack apparently was launched in the middle of President Trump's dinner with Chinese President Xi, and is not likely to make the Chinese very happy.) What legal authority for the use of force will President Trump assert under domestic and international law?

As a matter of domestic law, President Trump presumably relied on his Article II powers as Commander-in-Chief. The White House will likely file a report consistent with the War Powers Resolution tonight or tomorrow explaining the authority the President is asserting. Congress has not enacted an Authorization to Use Military Force against Syria. President Trump presumably decided that he did not want to ask Congress for authorization, in order to preserve the element of surprise and to avoid a delay or possible rebuff. His approach contrasts with President Obama, who as a candidate said that “The President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation.” When President Obama asked Congress to authorize the use of military force against Syria after Assad used chemical weapons in 2013, Congress failed to act.

As a matter of international law, President Trump does not have clear authority to use force in response to Syria’s use of chemical weapons, and he may not care whether U.S. actions are lawful under international law. Under the U.N. Charter, the U.S. is prohibited from using force in Syria unless authorized by the Security Council or exercising its right to individual or collective self-defense. The U.S. Government—like most other governments (with the exception of the United Kingdom and Denmark)—has never recognized a right of humanitarian intervention under international law. China and Russia have continued to block Security Council resolutions that would authorize the use of force to respond to the Assad regime’s atrocities. Yesterday, U.S. Ambassador to the U.N. Nikki Haley suggested that the U.S. might use force without U.N. approval: “When the United Nations consistently fails in its duty to act collectively, there are times in the life of states that we are compelled to take our own action.”

Having proceeded without U.N. authorization, it would be better if the Administration attempts to explain its actions as lawful, or at least justified, under international law. Although a U.S. military strike cannot be easily justified as self-defense of the United States, it is possible that the United States could argue that the use of force was permissible as an action in collective self-defense of Syria’s neighbors. Alternatively, it might be better for the Trump Administration to argue that its limited use of force was justified, even if not strictly lawful, under international law based on the specific facts in Syria and that other avenues had been exhausted. This is the approach the Clinton Administration took when it participated in the NATO bombing campaign in Kosovo in the 1990s, and that the Obama Administration was apparently prepared to take if it had decided to use force against Syria in 2013.

Last November, I argued in my Lloyd Cutler Rule of Law Lecture that “It will be critical for President Trump, Vice President Pence, and their senior advisers to learn and follow domestic and international law governing the use of force…. Should President Trump decide to use force in another country for a humanitarian purpose without approval of the Security Council, he should be prepared to explain when and why the use of force is legitimate under certain limited conditions, even if not clearly lawful under international law.”