Obama's decision to join the effort to enforce the U.N. Resolution against Libya seems to be justified by something called the Responsibility to Protect Doctrine.
From Wikipedia:
The responsibility to protect can be thought of as having three parts.
1.A State has a responsibility to protect its population from genocide, war crimes, crimes against humanity and ethnic cleansing (mass atrocities).
2.If the State is unable to protect its population on its own, the international community has a responsibility to assist the state by building its capacity. This can mean building early-warning capabilities, mediating conflicts between political parties, strengthening the security sector, mobilizing standby forces, and many other actions.
3.If a State is manifestly failing to protect its citizens from mass atrocities and peaceful measures are not working, the international community has the responsibility to intervene at first diplomatically, then more coercively, and as a last resort, with military force.
- Daniel Larison is not convinced by it.
- Some detail on how the decision was made to go at Libya.
- International Coalition for the Responsibility to Protect.