hi! not discouraging new contributions, but FYI, there was a very popular thread on this topic last year which will give you a great head start

Did the Germans and Japanese have names for the Americans along the lines of "Krauts" and "Japs"?

While your question is very interesting, we'd wish you to repost this with a specific conflict in mind (i.e. WWI, Korean War, etc.) to fit within our rules.