The Second Balkan War of 1913 was the result of Bulgaria not getting the lands it was promised in the pre-war agreement and going to war for them with the Serbs and the Greeks over it, with the Romanians and the Ottomans joining in against Bulgaria later on. The Bulgarian invasion plan was poorly thought out and since Bulgaria took several actions that agitated Russia, the Russian Empire dropped its support for the Bulgarians during the war. Bulgaria was, not surprisingly, defeated.
However, what if several things turned out differently, leading to a Bulgarian victory in the war?
The Bulgarians appease their Russian allies in order to get Russia's assistance during the war. They do this by promising to not attack Constantinople (which Russia wanted), and by doing their best to cooperate with Russia's attempts to make peace between them and the Serbs (or at least making a good appearance of doing so). When the war began, Russia put pressure on Romania to stay out of the conflict and provided military assistance. The brilliant Nikola Zhekov, a Bulgarian army officer, did not get sick during the summer of June 1913 and was able to take part in the war, making a good plan and carrying it out with significant success.