My friend and I were musing about this in light of the airstrikes launched against the Assad regime yesterday and we realized we can't come up with one except perhaps the Korean War or the Bosnian War, if one considers those civil in nature.
"Successful," in this sense, would mean short- and long-term betterment of the situation of the average person in that country, not success in whatever the US's objectives were.
One final note--I've read the subreddit rules and recognize this question may be considered too open-ended or soapboxing. I've attempted to limit the scope of my question by era (1900-1997), actor (the US), and type of conflict (civil) in order to narrow the scope. Furthermore, I'm not attempting to stir a hornet's nest by suggesting that any intervention is predestined to failure or anything like that--in all honesty, the question is borne out of simple curiosity. Thanks!
Operation Power Pack- The US invasion of the Dominican Republic accomplished most of its objectives, put the loyalists in power, and is usually considered successful. Operation Urgent Fury- The US invasion of Grenada reinstated the government and forced out the Cubans and Soviets. Operation Classic Resolve- The US helped to defeat a coup attempt in the Philippines successfully
There are others but these three were the first to come to mind for me and seem to be within the scope of your question. I hope this helps.