The Second Balkan War was a conflict that broke out when Bulgaria, dissatisfied with its share of the spoils of the First Balkan War, attacked its former allies, Serbia and Greece, on 16 (O.S.) / 29 (N.S.) June 1913. Serbian and Greek armies repulsed the Bulgarian offensive and counterattacked, entering Bulgaria. With Bulgaria also having previously engaged in territorial disputes with Romania and the bulk of Bulgarian forces engaged in the south, the prospect of an easy victory incited Romanian intervention against Bulgaria. The Ottoman Empire also took advantage of the situation to regain some lost territories from the previous war. When Romanian troops approached the capital Sofia, Bulgaria asked for an armistice, resulting in the Treaty of Bucharest, in which Bulgaria had to cede portions of its First Balkan War gains to Serbia, Greece and Romania. In the Treaty of Constantinople, it lost Adrianople to the Ottomans.

The Tsardom of Bulgaria, also referred to as the Third Bulgarian Tsardom, sometimes translated in English as the "Kingdom of Bulgaria", or simply Bulgaria, was a constitutional monarchy in Southeastern Europe, which was established on 5 October 1908, when the Bulgarian state was raised from a principality to a tsardom.