In my opinion, America is great because of its allies. Look at the war going on between America and Vietnam. At least 2.7 million Americans on duty from Vietnam recorded 58,159 deaths, 1,719 missing, and 303,635 wounded (wikipedia).
So also if there is war between America with China or North Korea. World War 3 is likely to happen soon after the end of the Cold War. Many countries will break with the birth of new countries.