Yglesias, after noting that mostly liberal New York Rep Anthony Weiner supports the Libya action by citing what we didn't do in Rwanda:

I think it's telling that enthusiasts for this kind of war typically have to make the case with reference to hypothetical success stories about military operations we didn't undertake. These are useful cases to deploy in arguments, because since the intervention didn't happen one doesn't need to wrestle with the potentially problematic consequences and downside risks. The key actual case is Kosovo, where American intervention was highly successful at helping the Kosovo Liberation Army achieve its political goal of independence from Serbia, but considerably less effective at actually preventing violence against civilians. What's more, though Kosovo independence is very nice for the Kosovar Albanians, it's hardly been a humanitarian boon to Kosovar Serbs and further afield it wound up creating problems for the good people of Georgia when Russia decided to use them as the target of retaliation for western recognition of Kosovo independence. We also have in Iraq and Afghanistan examples of military undertakings where the welfare of the inhabitants of the soon-to-be-bombed country was cited as a pro-bombing argument, and yet the actual results have been pretty mixed.