1915: October 11
Both sides had been secretly attempting to recruit Bulgaria but it was only on October 11, 1915 that the country announced its decision to join the Central Powers by declaring war on Serbia.
Bulgaria mobilised quickly, hoping to annex the Serbian province of Macedonia, and by October 21 its troops had taken the Macedonian capital, Skopje. In doing so the army had driven a wedge between the Allied forces in Greece as they tried to reach their Serbian allies.
By the summer of 1916 Bulgaria had launched a major offensive against neutral Greece, invading and occupying a section of the country. Their advance was only halted by a series of British aerial and naval bombardments. A stalemate ensued until 1918 when Greek and Allied forces pushed the Bulgarians into retreat.