World war 1 broke out in 1914 because a group of Serbian nationalists (The Black Hand) assassinated the duke of Austria-Hungary. Austria -Hungary threatened Serbia with a bunch of wishes that they wanted Serbia to due for them. Due to the treaty between Serbia and Russia, Serbia said no. Germany, who had a treaty with Austria-Hungary threatened to attack if Russia mobilized its army. Russia did and Germany invaded Belgium. This brought Great Britain and France into the war.