Countries are so diverse. They have different holidays and cultures, their geography differs, the language they speak is defferent. They also govern their countries differently and have different resources naturally available to them in their land. Especially those last two reasons are why countries don't get along sometimes. And when they REALLY don't get along, war can start.
When war starts between two countries, their allies often will help finance the war, provide them with weapons, or even send troops to help fight. When many countries start doing this, it becomes a world war. In order to create a world war, there needs to be at least two sides involving at least two countries that then get their allies involved in the war.
I think that another world war today is very possible. The U.S. doesn't agree with how other countries are run, for example, North Korea, and if North Korea did something we didn't agree with, we could start war with them. Depending on what other countries want, they could decide to join sides and help fight. North Korea was just an example though. We are also very dependent on other countries for resources such as oil. When coutries are so dependent and desparate for such resources, they may be greedy and drastic with their actions. If our import of resources was for some reason denied or anything got in the way of our agreement with other countries on that, we could intervene and start war. Once again though, depending on the opinions of other countries, it might not turn into a world war. It depends on the web of alliances and how many countries choose to get involved. Another world war in the future is very possible though.
