Catching the Entente by surprise, Bulgarian forces pushed the Serbs out of Macedonia and into Albania and occupied part of Greek Macedonia by mid-1916. British, French, and Serbian troops landed at Salonika and stopped the Bulgarian advance, but the Entente's holding operation in Greece turned into a war of attrition lasting from late 1916 well into 1917. This stalemate diverted 500,000 Entente troops from other fronts. Meanwhile, Romania had entered the war on the Entente side in 1916. Bulgarian and German forces pushed the poorly prepared Romanians northward and took Bucharest in December 1916. The Bulgarians then faced Russia on a new front in Moldavia (the part of Romania bordering Russia), but little action took place there.

Data as of June 1992