Why did Serbia enter World War 1?
When Austria-Hungary's heir to the throne, Archduke Franz Ferdinand was assassinated by Gavrilo Princip, a member of the 'Black Hands', Austria-Hungary, who were feeling confident due to the support Germany had promised them, demanded Serbia to hand over the assassin together with the leaders of the 'Black Hands'. When Serbia failed to do so, Austria-Hungary declared war on Serbia.
WW1 actually started as a conflict between the small country of Serbia and Austria-Hungary because Serbia supported the assasination of the Archduke or Austria-Hungary. Because Serbia was in an alliance with Britain, it had an obligation to enter the war, along with many other countries that entered on both sides of the intricate system of alliances that made up Europe and caused World War 1.