I actually think the U.S. would be far less squeamish than people think.

Since the Vietnam War we've had really only one justified conflict (Kuwait). It was a short war to defend a country. From everything I've heard, opposition to the war was small, especially compared to our two most recent debacles.

What the U.S. loathes is a pointless war. There needs to be a goal and that goal needs to be achievable. World War 2 is a great example of a war with a point: stop Hitler, defeat Japan. The goal is accomplished in visible ways, and the war itself isn't one of U.S. Aggression.

If China invades Japan, then we're back in to a war of defense. Not defending vague concepts, but sovereign nations. It'll be precisely the sort of war the U.S. is best at; overseas, formal, and military on military rather than military on civilians. I honestly think the population would rally to the cause and we'd have full on World War 3.

Remember, the one thing that Americans can do better than anyone else on the planet is any single task they put their minds to. Get to the moon first? Done. Supply a war in Europe? Got it. Electronics? We'll make it work. Internet? Sure. If you can get the United States to focus itself on a single task that task will be done and be done well. The problem is getting that focus.