The law changed as governments broke the old rule and asserted a new one. This is the international version of seeing the U.S. Constitution as a "living document" - it must be read in light of today's understandings.
To intervene lawfully in Syria, the U.S. will have to take the view that international law has changed since the U.N. Charter was written in 1945. As new challenges arise, the old rules are reinterpreted and remade. This makes it difficult to make definitive judgments about what is legal or illegal. For some, it may also clear a "legal" path for the U.S. to justify military action to defend the Syrian people from Assad.
Ian Hurd is associate professor of political science at Northwestern University and the author of "After Anarchy: Legitimacy and Power in the UN Security Council," "International Organizations: Politics, Law, Practice" and other works on the politics of international law. He is @ian_hurd on twitter.