A war is classified as a world war when a large number of countries spanning multiple continents are involved in the conflict. A better way of answering what classifies a world war may be to answer the question of what is a war. A war is a conflict between two nations, empires, ruling powers, or tribes. The key is that a war is between 2 parties while a world war involves many nations. Which also poses the question of why are the Korean and Vietnam wars not considered World Wars. The answer is quite simple the Korean war and the Vietnam war were "police actions" where major countries like the United States and the Soviet Union did not directly declare war on one another, but were only involved in the conflicts as a means of helping other nations.