One of the great things about capitalism is that people would rather sell things to each other than fight, and that's coming from a socialist.
Also, it is very difficult for democracies to fight each other (the only one I can think of is the American Civil War, and its a stretch calling the South a democracy with less than 20% of the population being able to vote). With the world as democratic as it is right now, I have a hard time seeing another World War unless a lot of social change happens.