The main causes of World War I were militarism, alliances, imperialism, and nationalism. Militarism involved the growth of military forces and an arms race among European nations. Alliances formed between countries, creating a complicated web of alliances that dragged multiple nations into the conflict. Imperialism led to competition for colonies and resources, increasing tensions among nations. Nationalism fueled a sense of pride and loyalty to one's country, often leading to conflicts and rivalries between nations.