One of the craziest things about World War I is the cause behind it. The very entangling alliances that George Washington warned the United States to avoid were the catalyst for worldwide war.
Franz Ferdinand was assassinated, and as each country declared war on the others, the situation rapidly got out of hand. Alliances came into affect which drew otherwise disinterested countries into the fray, finally engulfing the entire world in a conflagration which would claim the lives of over 8 million people.
As a result, the United States was hesitant to enter the second World War, until finally provoked by Japan. The same causes of the first world war were to surface again, however; Russia helped bring the war in the Pacific to an end by declaring war on Japan, as Japan was an ally of Nazi Germany.