After the outbreak of the First World War, Greek-Macedonia found itself in the eye of the storm, as both warring parties -- the Central Powers (Germany and Austria-Hungary) and the Entente (initially Great Britain, France and Russia) -- used it as bait in their efforts to lure Bulgaria on their side.
After the Ottoman Empire's entry into the war in 1914, and Bulgaria's entry in 1915, both on the side of the Central Powers, the Entente landed troops at Thessaloniki. In 1916, Sofia, resentful of its losses in Macedonia by the Treaty of Bucharest, occupied eastern Macedonia and part of Serb-Macedonia.
Meanwhile, in Greece the National Rift erupted between the Prime Minister Eleftherios Venizelos and King Constantine, when the latter did not allow the former to bring Greece into the conflict on the side of the Entente. The Rift culminated in the setting-up of two Greek states: one in Athens under Constantine, the other in Thessaloniki under Venizelos.
Venizelos inspects the Greek units
National Historical Museum, 1918
The Rift ended in 1917, when the National Defense Government of Thessaloniki prevailed. The country was then reunited and took part in the operations on the Macedonian Front. In 1919-20, Greece made substantial territorial gains in Thrace and Asia Minor; however, with the exception of western Thrace, it failed to keep them.