Which of the following scenarios do you think would be a justified military intervention by the US?

[Schiff for Senate]

Definitely #1 and #2.

#3 definitely if the country that is the victim of the terrorist attack is America, and maybe if it is not America (depending on the context of the situation.)

#4 might be acceptable. It probably is, if the reason is paired with one of #1, #2 or #3, but if the sole cause of going to war is residents of some other country being oppressed, we should be very careful about which ones we choose to entrench ourselves in. If America went to war over every country with a brutal/unfair leader, we wouldn't be able to do anything else and we'd lose at least some of the wars.