The First World War happened because of several reasons:
1. Nationalism: People were passionately proud of their countries , but some groups demanded separation and independence, which led to conflicts and violence.
2. Imperialism: Countries like Britain, France, and Germany wanted to control more land and colonies around the world, leading to arguments and rivalries.
3. Militarism: Countries built big armies and weapons and were ready to fight at any moment, which made war more likely
4. Alliances: Countries made secret promises to help each other. So, when one country was attacked, others jumped in, making the war bigger.