Dear student,
During the eve of the First World War, the Balkans were the scene of conflict between Serbia and Austria Hungary. During the early part of the 20th century, Austria Hungary had annexed Bosnia Herzegovina, a territory in the Balkans. Bosnia had a substantial Serb population. So Serbia, with the aid of its ally, Russia, actively aided the independence movement in Bosnia against Austria Hungary. This created animosity between Serbia and Austria Hungary.
The tension between Serbia and Austria Hungary led to the assassination of Archduke Franz Ferdinand, the prince of Austria Hungary, on June 28, 1914, in Sarajevo, the capital of Bosnia Herzegovina. The assassin was a Bosnian Serb. Angered by the assassination, Austria Hungary declared war on Serbia. Serbia's ally, Russia, then declared war of Austria Hungary. This was the beginning of the First World War.
Regards