Via Jens David Ohlin, the US case for air strikes in Syria:
So the structure of the argument goes as follows. The right of response is originally Iraqi, and the U.S. right of intervention is parasitic upon the Iraqi claim. Iraq has been attacked by ISIS, thus triggering Iraq’s right of self-defense against ISIS. Furthermore, since Syria is apparently unable to adequately respond to the ISIS threat and prevent its forces from using Syria as a base of operations to launch attacks against Iraq, then Iraq is entitled to use military force against ISIS installations and forces in Syria, even without the consent of the Syrian government or authorization from the Security Council. In other words, this falls under the inherent right of self-defense that is carved out by Article 51 of the U.N. Charter as an exception to the general prohibition on the use of force contained in article 2 of the U.N. Charter. The U.S. is intervening militarily to vindicate Iraq’s self-defense interest as a case of individual or collective self-defense.
My prediction : dark times ahead for international law and more dubious and ineffective international interventions. Is it just me or isn't there more silence around Ukraine all of a sudden?