A major topic in current events is the questionable legality of any potential move by the United States to intervene in the ongoing Syrian civil war. For those interested in better understanding this issue by reviewing the state of the laws governing the use of force, here are some resources that may be useful:
Article 2(4) of the United Nations (UN) Charter pledges member states to refrain from the threat or use of force against other states. However, there are two exceptions: Articles 34 and 51 permit the use of force when authorized by the UN Security Council or in self-defense, respectively. Although these articles constitute the law on the international use of force, many modern legal theorists have nonetheless developed justifications for humanitarian intervention when the UN refrains from authorizing the use of force for political or procedural reasons rather than declining to do so on the merits of a given case.
These justifications were used in 1999 when the North Atlantic Treaty Organization (NATO) intervened in Serbia to put an end to attacks on ethnic Albanians in Kosovo. In the case of Kosovo, the UN Security Council vetoed the use of force, but NATO’s actions were condoned by the international community despite the fact that the intervention did not accord with international law. The case of Kosovo is seen by some as a precedent for similar intervention in Syria: both situations involve similar accusations of domestic war crimes, and both are in a legal grey area created by objections on one side that the use of force without UN authorization would be illegal, and objections on the other side that the Security Council veto has broken the process that could result in UN authorization.
A legal analysis of this issue is beyond the scope of this posting; fortunately, legal scholars elsewhere have already done an excellent job. Anyone looking for an informed consideration of international law regarding the unauthorized use of force may be interested in essays posted here, here or here.