What could possibly create a world war?

In order of a modern day world war to happen something along the lines of a nuking of Isreal or britian would have to happen. The reason it would be something like launching a nuke at a country is that the world would have to take action against the country or group of countries that launched said nuke. Another reason this would create a world war is that America and other countries have strong connections with them. This is one of the only ways that a 3rd world war would happen