thumb|upright=1.1|Allied collaboration: an Italian captain, a Russian lieutenant, a Serb colonel, a French lieutenant, and a Greek gendarmeThe Allied Army of the Orient (AAO) (French: Armées alliées en Orient) was the name of the unified command over the multi-national allied armed forces on the Salonika front during the First World War.
When Germany, Austria-Hungary, and the newly joined ally, Bulgaria, were about to overrun Serbia in September–October 1915, the returning multi-national troops from the failed Gallipoli campaign disembarked in the Greek port of Salonika to establish the Macedonian Front. A side-effect of the landing was the further burdening of the National Schism between Constantine I of Greece and the Prime Minister Eleftherios Venizelos, ending in the forced resignation of the latter.
By August 1916, some 400,000 Allied soldiers from five different armies occupied the Salonika front. A unified command imposed itself and after long discussions, French General Maurice Sarrail was placed in command of all Allied forces at Salonika, although they retained right of appeal to their governments.
Greece itself remained at first neutral. After a coup on 30 August 1916, the Provisional Government of National Defence, led by Eleftherios Venizelos, was created in Salonika. It started assembling an army and soon participated in operations against the Central Powers. In June 1917, after increasing pressure from the allies, King Constantine I of Greece was forced to abdicate from the throne. Venizelos assumed control of the entire country and Greece officially declared war against the Central Powers on 30 June 1917. The Greek forces also operated under command of the AAO.
The Serbian armies were corps sized formations.