What peacekeeping missions have the US forces participated in?

A complete answer would take more time than I have available, but see related link "United Nations Peacekeeping Missions, Past and Current" at the bottom, where you can find facts, figures (including contributing nations) for all United Nations peacekeeping operations. Bear in mind that peacekeeping operations are not solely a function of the United Nations, but have been conducted by the North Atlantic Treaty Organisation, the African Union, and the European Union.