It is actually rather poorly-charted territory in international law, and the basis for either us, OR NATO, OR the UN, to step in is open to some serious question. It fails as far as "International armed conflict" goes, and also as "Genocide," since Daffy Quadaffy clearly is using force directed at political rebels, not against an identifiable ethnic, religious, or racial group on any basis except quelling armed resistance, so in that respect it is completely unlike the primary justification for Kosovo. Sierra Leone is probably the closest analog, with the French-led UN intervention there, but I believe there is a plausible genocide argument in that one (Sierra Leone, by the way, is another neverending cluster****, for the information of anyone unfamiliar with it).