World War I was caused by militarism, alliances, imperialism, and nationalism in Europe. Militarism led to an arms race between countries as they built up their militaries. Alliances pulled many countries into the war once it began as countries were bound by treaties to support their allies. Imperialism and competition for colonies intensified rivalries. Nationalism heightened tensions as countries sought to create or break from empires and ethnic differences grew between nations. The assassination of Archduke Franz Ferdinand by a Serbian terrorist led Austria-Hungary to declare war on Serbia, pulling the rest of Europe into the war through their alliances.