In my opinion, it will result from the United States getting too nosy in foreign conflicts (either CIA or Israel), drawing the West into a smaller war, which will then escalate, possibly with other somewhat powerful countries backing the other side. With increasing globalization, some parties may have connections around the world, resulting in terrorism that brings the fight to home soil. This will be the beginning of a full war.
in other words, US intervenes, terrorists bring the war to an already rather politically-charged west, bam. war.
