With Montenegro and Serbia engaged in the First World War from its outbreak, three other southeastern European countries hovered on the periphery of the fighting: Bulgaria, Romania, and Greece. Since, like Italy, all three harbored desires to realize their nationalist aspirations at the expense of their neighbors, they could not ignore the opportunities presented by the war. With the fighting raging elsewhere in Europe between the two great power alliance systems, affiliation with one or the other side promised substantial benefits. Before acting, each remaining Balkan nation had to determine which side offered the greatest gains and the most likely chance of victory. At the same time, the two warring alliance systems, the entente (France, Great Britain, and Russia) and the Central Powers (Austria-Hungary and Germany) sought any possible advantage in the Balkans. The interest of both blocs in this region increased after the fighting had deadlocked elsewhere.

Bulgaria, Romania, and Greece, like their Serbian neighbor, had cast off Ottoman rule in the nineteenth century. All three regarded their borders as temporary, because significant numbers of their conationals lived close by under foreign rule. All three devoted much of their national energy and treasure toward the establishment of large national states based on historical and ethnic claims, on occasion resorting to war for this purpose. Bulgaria fought Serbia successfully to this end in 1885; Greece the Ottoman Empire unsuccessfully in 1897. In the recent Balkan Wars of 1912–13, all three countries participated to further their nationalist aims.