How Did Bulgaria and Turkey End Up on the Same Side in WWI?

Turkish and Bulgarian Border Guards

The greatest moment in Bulgarian history came when Bulgaria proclaimed its independence from the Ottoman Empire on 22 September 1908. Its earliest strategic objective was to complete unification with Bulgarian peoples still under Ottoman rule. Hence, their subsequent joining the Balkan League to take on the Ottoman Empire in the First Balkan War. Results of this first war were positive for Bulgaria, but the treaty which followed resulted in a quarrel over the spoils of war. Bulgaria, dissatisfied with its share of the conquests of the First Balkan War, attacked its former allies, Serbia and Greece, in June 1913. It lost this time. The outcome of this  Second Balkan War negated almost all of the territorial gains that Bulgaria secured during the First Balkan War.  

At the end of  September 1913, Bulgaria—big loser of the Second Balkan War—decided to negotiate with the Turks—big loser of the First Balkan War—directly. The terms of the Treaty of Constantinople they negotiated provided that defeated Bulgaria would agree to Turkey's repossession of Adrianople (Edirne), plus territory up to the Maritsa River. In addition, the two countries agreed to resume diplomatic relations, exchange prisoners, and establish a general amnesty. The former adversaries—both embittered by their experience in the small wars of the 1910s—would find themselves allied with the Central Powers in the Great War, hoping to reverse their declining fortunes.