Here are two examples as to the results when the US i ntervenes: In Korean War the US succeeded in stopping the communists from invading the south. The results? The north became and has remained a third world nation with it's people living in misery while south Korea is an economic power-house with a very high standard of living. In the Vietnam War the US failed to stop the communists from conquering the south. The result? The GNP of Vietnam and the standard of living of it's citizens went down the crapper after the south fell and Vietnam has been a third world shit-hold place of a country. I'm not even going to go into what the US did during WWII because that goes without saying the sacrifices made for other nations.
Is the US perfect? Nope. But before you imply that the USA as a selfish, war-monger nation think about the conflicts we have been in. Do you know what the countries in the middle-east really fear about the US going to war with Iraq? They worry that a democracy will be set up and this spirit of freedom will spread to other countries in the region.