Obama’s caution about intervening directly in Syria or arming the opposition has been prudent. Intervention without an international legal basis could make it more difficult for Washington to criticize other countries if they intervene in neighboring states based on less laudable motives. Inserting more arms into an already unstable region risks more bloodshed, and those weapons could fall into the hands of groups hostile to U.S. interests, as happened in Libya.

As the violence in Syria increases, however, the president is likely to feel compelled to provide more than political support and non-lethal aid. If the Syrian Opposition Council becomes more inclusive and can legitimately claim to represent the majority of Syrians, and if it excludes terrorist groups and other extremists, the administration may conclude that it is legally permissible to provide military assistance based on the council’s consent. But other countries may disagree with this rationale. Alternatively, the administration could intervene in a limited way to protect civilians without asserting a legal basis, as the Clinton administration did with its participation in the 1999 NATO bombing campaign of Kosovo, to protect Kosovars from atrocities committed by Serbia. The Kosovo intervention was viewed by most international lawyers as legitimate, although not strictly legal.

Humanitarian crises challenge international legal rules as well as our consciences. But when the Security Council is blocked from protecting civilians against the most egregious atrocities, the United States should be prepared to intervene when other avenues have been exhausted and there is sufficient international consensus to support intervention. If Assad’s attacks on Syrian civilians continue, the United States and other governments may soon conclude that intervention is morally, if not legally, justified.

John B. Bellinger III is a partner with Arnold & Porter LLP and an adjunct senior fellow in international and national security law at the Council on Foreign Relations. He served as legal adviser to the State Department from 2005 to 2009.