The Balkans Campaign, or Balkan Theatre of World War I was fought between the Central Powers, represented by Austria-Hungary, Bulgaria, Germany and the Ottoman Empire on one side and the Allies, represented by France, Montenegro, Russia, Serbia, and the United Kingdom (and later Romania and Greece, who sided with the Allied Powers) on the other side.
The prime cause of World War I was the hostility between Serbia and Austria-Hungary. Consequently, some of the earliest fighting took place between Serbia and Austria-Hungary. Serbia held out against Austria-Hungary for more than a year before it was conquered in late 1915.
Dalmatia was a strategic region during World War I that both Italy and Serbia intended to seize from Austria-Hungary. Italy entered the war in 1915 upon agreeing to the Treaty of London that guaranteed Italy a substantial portion of Dalmatia.