What Creates a World War? What Would Create One Today?

I think that a world war is created by one extreme act by one country. Then another country steps in to protect the people that are affected by that act. Eventually it will snowball into a world war because the other countries that are allied with the original countries will step in to help. With all of these countries fighting on one side or the other it creates a world war. I believe that a world war could be created today if other countries went to the aide of the middle eastern countries fighting the U.S. This would force the U.S to get back up from our allies causing a world war.