With Montenegro and Serbia engaged in the First World War from its outbreak, three other Balkan countries, Bulgaria, Romania, and Greece, hovered on the periphery. Because, like Italy, all three desired to realize their nationalist aspirations at the expense of their neighbors, they could not ignore the opportunities presented by the war; affiliation with the Triple Entente or the Central Powers promised substantial benefits. Before acting, each Balkan nation had to determine which side offered the greatest gains and the most likely chance of victory. Conversely, the two warring alliance systems (Triple Alliance and Triple Entente) looked to their possible advantage in the Balkans.

These three nations like their Serbian neighbor, devoted much energy and treasure toward the establishment of large national states based on historical and ethnic claims. Bulgaria fought Serbia successfully to this end in 1885; Greece fought the Ottoman Empire unsuccessfully in 1897. In the Balkan Wars of 1912–13 all three countries participated to further their nationalist aims. The victories of Greece and Romania, and the defeat of Bulgaria, only intensified nationalist strivings. In the aftermath of the Balkan Wars, Greece and Romania sought to expand their gains; the Bulgarians to obtain revenge and to restore their national aspirations.

Since 1895, Bulgaria enjoyed the patronage of Russia. From 1882, Romania had a formal alliance with Austria-Hungary, and through that arrangement with Germany and Italy.