The Bulgarians adopted an advanced democratic constitution, and power soon passed to the Liberal Party led by Stefan Stambolov. Prince Alexander had conservative leanings, and at first opposed Stambolov's policies, but by 1885 he had become sufficiently sympathetic to his new country to change his mind, and supported the Liberals. He also supported the Unification of Bulgaria and Eastern Rumelia, which was brought about by a coup in Plovdiv in September 1885. The Powers did not intervene because of the power struggles between them. Shortly after, Serbia declared war on Bulgaria in the hope of grabbing territory while the Bulgarians were distracted. The Bulgarians defeated them at Slivnitsa and used the momentum to launch a counterattack. The Serbian army was pushed deeply into Serbian territory, but Bulgaria was forced to halt its advance after the Austro-Hungarian Empire threatened to intervene on the Serbian side. The unification was accepted by the Powers in the form of personal union.