There were many reasons why the First World War happened, I would sum it up to Nationalism, Imperialism, and All around tension. Many countries wanted to test their new technology considering it had been a few years since the last big war. There were mutual defense pacts that the European powers signed amongst one another and when Archduke Franz Ferdinand was assassinated, AH gave Serbia a purposefully impossible list of demands. When they declined, AH declared war. Serbia called Russia to arms and Russia pulled the French in. AH pulled Germany into the war and after the invasion of Belgium, The British Empire entered too.