On September 6, 1885, against the will of Russia and most of the other Great Powers, Bulgaria and the semi-autonomous Ottoman republic of Eastern Rumelia declared their unification in the city of Plovdiv.

The Austro-Hungarian Empire was particularly worried about this course of events as it was in the process of expanding its own influence in the Balkans at the time. Therefore, the Austrians induced the Serbians to attack the fledgling Bulgarian state, promising them territorial gains in return for concessions in the West Balkans. Serbians were hoping to have surprise on their side as Bulgaria was naturally expecting retaliation on the part of the Ottomans from the South. Bulgaria was additionally weakened because as the result of the Bulgarian-Russian conflict (caused partly by the Russian opposition to the unification) most Russian officers serving in Bulgaria had been withdrawn and Bulgaria had no officers above the rank of major.

Fortunately for the Bulgarians and the peace on the Balkans, the Ottomans did not choose to retaliate and the Serbian army did not succeed, as they were held at the Battle of Slivnitsa until the main body of the Bulgarian army made it in record time from the other end of the country to defend the capital.

After the defence battles at Slivnitsa and Vidin Bulgaria began an offensive taking the cities Pirot and Niš; at this point the Austro-Hungarian Empire warned that if the Bulgarian army didn't retreat it would join the conflict.

No territorial changes were made to either country and the Bulgarian unification was recognized by the Great Powers but the relationship of trust between Serbia and Bulgaria, built during their common fight against the Ottoman rule, suffered irreparable damage.