The four main causes of World War I were militarism, alliances, imperialism, and nationalism. Militarism refers to the belief that a country should maintain a strong military capability and be prepared to use it aggressively if necessary. Alliances were formed between countries in order to provide mutual defense and security, but this also created a complex web of obligations that made it difficult to avoid conflict. Imperialism involved the competition between European powers for colonies and resources around the world. Nationalism was the belief that one's own country or ethnic group was superior to others, and this often led to tensions and conflicts between different groups. These four factors contributed to the complex web of relationships and tensions that ultimately led to the outbreak of World War I.