Before the strikes late on the night of September 22, Eastern time, Obama was sure he had the domestic legal authority to hit Syria—he just had no explanation for why such a strike would pass the global test.
The White House has an answer for critics who want to know how the Obama administration can justify striking ISIS inside Syria under international law: If and when we actually do it, we will come up with a legal justification then.
The Obama administration has explained at length why it believes it has the domestic legal justification for using airstrikes in Syria; they have claimed they don’t need Congressional authorization because the 2001 authorization for the use of military force against the perpetrators of 9/11 and the 2002 authorization to take down Saddam Hussein applies to the ISIS war. The New York Times called the explanations “perplexing” and insufficient. (After all, al Qaeda and ISIS have sporadically fought with one another, and the Saddam regime is long gone.)
But the administration has said almost nothing about why airstrikes in Syria would not be a direct violation of the international law of armed conflict and the United Nations charter, as both the Syrians and their Russian allies have claimed.