Bulgaria joins the war

Bulgaria - under its almost insanely ambitious monarch - joins the war. Serbia is defeated.

Bulgaria felt cheated from its gains during the Balkan Wars. It wanted more. It DESERVED more. In that case, persuading its very pro-German monarch to join the Central Powers was merely the matter of time and money (he required credits to finish preparations of his army to its participation in the Great War).   When it finally started, Serbia was doomed. Attacked from two sides, its defenders were either defeated and returned home or fled to Albania, from which they were evacuated to Greece and continued waging war against Central Powers from there.