 The conflict quickly attracted the involvement of all major European countries, pitting the Central Powers against the Entente coalition, and starting World War I.

Serbia was ultimately defeated during the autumn 1915 phase of the Serbian Campaign, prompting France and Britain to transfer troops from the Gallipoli Campaign to Greek Macedonia.The Macedonian front was thus established to support the remnants of the Serbian army in their effort to conquer Vardar Macedonia.On 17 August 1916, Bulgaria invaded Greece, easily conquering all Greek territory east of the Struma, since the Greek Army was ordered not to resist by the pro-German King Constantine.