What was the Cause of World War One?

World War I was caused by alliances, nationalism, militarism, and imperialism. The nations of Europe sought to increase their wealth and power by expanding their reach to foreign shores, and sought to protect these possessions and riches by forming alliances. When a Serbian nationalist assassinated Austrian Archduke Franz Ferdinand, the Austrians (allied with Germany) declared war, causing the Russians (allied to the United Kingdom and France) to defend Serbia.