On September 6, 1885, revolutionaries in Plovdiv overthrew the Ottoman-appointed governor of Eastern Rumelia and declared union with the Principality of Bulgaria, defying the 1878 Congress of Berlin's partition of Bulgarian lands. Prince Alexander of Battenberg endorsed the union days later, triggering a brief war with Serbia but ultimately securing a unified Bulgarian state recognized by the Great Powers.
The Congress of Berlin in 1878 had split the Bulgarian lands liberated from Ottoman rule into an autonomous principality in the north and a nominally Ottoman but internationally supervised province, Eastern Rumelia, in the south — a compromise designed by the Great Powers to prevent Russia from dominating a large unified Bulgaria. Bulgarian nationalists never accepted the division.
On the night of September 5 into September 6, 1885, a group of revolutionaries and sympathetic army officers in Plovdiv seized the Eastern Rumelian governor-general and proclaimed unification with Bulgaria, presenting the Great Powers with a fait accompli. Prince Alexander, initially cautious about provoking Ottoman or Russian anger, accepted the union under popular pressure.
The move provoked Serbia to declare war, fearing Bulgaria's growing power would upset the regional balance, but Bulgarian forces repelled the Serbian invasion within weeks. The unification stuck, and the episode is remembered in Bulgaria as a defining act of national self-assertion against great-power diplomacy — September 6 is still celebrated as Unification Day.