Hmm.. how recent are we talking? US Foreign policy has been dominated by 9/11 and the war in Iraq recently.
If you can stretch it that far, the intervention in Bosnia is usually considered a good thing.
Apart from that it should be easy enough to talk about how liberating Iraq from an oppressive dictator is a good thing, especially for the minority groups. As well as liberating the people of Afghanistan from the ultra-conservative taliban regime.
Though for Iraq, the US has taken a lot of criticism so it would be better to argue about another case if you can.
hmm... also the current circumstance in Darfur might be useful. It's not somewhere the US has actively engaged in yet, but it is an area where intervention could be argued to be a good idea. Whether militarily or politically.
Were I writing it I would go with the taliban and dahfur. That would give you an oppertunity to show a sucessful intervention, and a place where internvention would be a good thing.
http://en.wikipedia.org/wiki/Darfur_conflicthttp://www.savedarfur.org/content?splash=yesSome useful information on the subject if you choose to go for that.