The Unification of Bulgaria (Bulgarian: , Saedinenie na Balgariya) was the act of unification of the Principality of Bulgaria and the province of Eastern Rumelia in the autumn of 1885. It was co-ordinated by the Bulgarian Secret Central Revolutionary Committee (BSCRC). Both had been parts of the Ottoman Empire, but the Principality had functioned de facto independently whilst the Rumelian province was autonomous and had an Ottoman presence. The Unification was accomplished after revolts in Eastern Rumelian towns, followed by a coup on 18 September [O.S. 6 September] 1885 supported by the Bulgarian Knyaz Alexander I. The BSCRC, formed by Zahari Stoyanov, began actively popularizing the idea of unification by means of the press and public demonstrations in the spring of 1885.

The Serbo-Bulgarian War or the Serbian–Bulgarian War (Bulgarian: Сръбско-българска война, Srăbsko-bălgarska voyna, Serbian: Српско-бугарски рат, Srpsko-bugarski rat) was a war between the Kingdom of Serbia and Principality of Bulgaria that erupted on 14 November [O.S. 2 November] 1885 and lasted until 28 November [O.S. 16 November] 1885. Despite Bulgaria being a vassal state of the Ottoman Empire, the Turks did not intervene in the war. Serbia took the initiative in starting the war but was decisively defeated. Austria demanded Bulgaria stop its invasion, and a truce resulted. Final peace was signed on 3 March [O.S. 19 February] 1886 in Bucharest. The old boundaries were not changed. As a result of the war, European powers acknowledged the act of Unification of Bulgaria which happened on 18 September [O.S. 6 September] 1885.