What creates a world war?

Anything can create a world war really. In the past there were really good reasons to fight and not so good ones but today there are a few good reasons why a world war could start. For example, if the U.S. was attacked again by Iraq or Afghanistan we could very well launch an attack of them and then other countries would get involved to either fight with us or against us. Another reason why a world war could happen is if resources start to become scarce. If there was a shortage of oil most countries would want to have that resource, so then if there are a bunch of countries wanting the same thing there can be a lot of tension created which could very well lead to another world war. Another example is if come countries claims they have weapons of mass destruction and threatens another country. The once again there's all this tension created.