How did world war 1 get so big

Difference in policies were to blame, although the immediate cause of World War one was the assassination of Austria's Archduke Franz Ferdinand. The war started mainly because of four aspects: Militarism, Alliances, Imperialism and Nationalism

The immediate causes lay in decisions made by statesmen and generals during the July Crisis of 1914. This crisis was triggered by the assassination of Archduke Franz Ferdinand of Austria by the Bosnian Serb nationalist Gavrilo Princip who had been supported by a nationalist organization in Serbia.