"If everything you do can be justified by some good results, then we really should impose a draft and invade one benighted country after another and improve things."
I think these benighted countries are safe from American invasion as long as they don't violate international law, such as Saddam's regime, where he invaded Kuwait, then after signing a cease fire agreement failed to follow the agreement. Or in the case of Afghanistan, having terrorist groups within our country which engage in major terror act and then refuse to take significant measures to assure that state is not harboring and/or supporting these terrorists. This last one is a bit of a grey area as far as international law, this is due past covert wars which many countries have been involved in. Where in draw the line is a bit uncertain, but it's pretty safe to say that flying airliners into buildings is going pretty far over any line. Another grey area in regards to international law is when the State "appears" to be engaged in genocide.

So if benighted countries will follow international law, make sure that groups within their countries aren't engaged in covert warfare, AND aviod anything which might appear to be genocide on there own people, then they are pretty safe from invasions or miltary act from America.