It was mostly European countries, but remember that this was the age of colonialism. Most of the world was owned by one European country or another. This includes basically all of Africa (by various countries), India (by the British), the East Indies (primarily by the Dutch, with a bit of Portuguese), Southeast Asia (several countries, but the French owned quite a bit of it), all the Middle East (Britain and France), all of the Pacific Islands (various countries), and parts of South America (France, Britain and the Netherlands). Since Britain was in the war, all of the commonwealth nations were drawn in as well - that means Canada, Australia, and a couple others. Also, Japan was an ally of the British and French. They attacked German ports in the Far East, and were part of the Treaty of Versailles. Put all of this together, and a huge amount of the world is involved in this war.