<human>:  Why did the war start?
<bot>: The war started in 1914, when Austria-Hungary declared war on Serbia. Austria-Hungary was a member of the Triple Alliance with Germany and Italy. This alliance had been in place since 1882, and was seen as a defensive pact against Russia. Serbia was a member of the Balkan League, which was an alliance of Balkan states. The alliance was formed in 1912, and included Greece, Bulgaria, and Romania.