Compared to other countries, the U.S. has always been very active in foreign policy, with varying success, and for good and bad. In the days of the Cold War, the main concern was the containment of communism, and consecutive U.S. governments (of both parties) were pleased to support gruesome right wing dictators all over the world (“he may be a bastard but he’s our bastard”).
However, after the collapse of communism in the 1990s, the U.S. began to understand that dictatorship and rights violations abroad, even of the non-communist type, are contrary to the national interest of the U.S., and also a moral problem in their own right. It is now generally accepted in the U.S. that the promotion of respect for human rights in other countries should be one of the major aims of foreign policy, and that intervention of some kind in the business of other countries can be justified if this is necessary for human rights (especially in cases of gross violations such as genocide).
Yet many in the U.S. worry about foreign “adventures”, even if human rights are at stake. Here is a poll showing decreasing levels of support for human rights as a foreign policy priority. And below is a very unscientific but funny measure of how much U.S. citizens care about the rest of the world:
Here and here are posts on humanitarian intervention, a type of intervention limited to the most extreme rights violations, often involving military force.