On , Bulgaria and the semi-autonomous Ottoman province of
Eastern Rumelia declared their unification in the city of
Plovdiv. Eastern Rumelia, whose population was predominantly ethnic Bulgarian, had been an artificial creation of the
Berlin Congress seven years earlier. The unification took place against the will of the
Great Powers, including
Russia. Austria-Hungary had been expanding its influence in the
Balkans and was particularly opposed to the unification. Bulgaria's western neighbour, Serbia, also feared it would diminish its position in the Balkans. In addition, Serbia's ruler,
Milan I (1868–1889), was annoyed that pro-Russian opposition leaders like
Nikola Pašić, who had stirred up the
Timok Rebellion, had found asylum in Bulgaria after the suppression of the rebellion by the Serbian Army. After the declaration of unification, massive protests broke out in
Greece for fear of the creation of a Greater Bulgarian state in the Balkans. They called upon the Greek government to declare war on Bulgaria. Serbia proposed to Greece a joint military action against Bulgaria, but Greece rejected the proposal. Lured by Austria-Hungary's promises of support and territorial gains from Bulgaria (in return for concessions in the Western Balkans), Milan I declared war on Bulgaria on . The military strategy relied largely on surprise, as Bulgaria expected an attack from the Ottoman Empire and had moved its troops to the area near the Ottoman border, to the southeast. The Serbian pretext was a minor border dispute, known as the Bregovo Dispute.
Timok, which formed part of the border between the two countries, was a river that had slightly changed its course over the years. As a result, a Serbian border guardhouse near the village of
Bregovo had found itself on the Bulgarian bank of the river. After some denied requests from Bulgaria to evacuate the guardhouse, Bulgaria expelled the Serbian troops by force. Bulgarian sources on the other hand, outlined several Serbian intrusions into Bulgarian territory as the start of hostilities. Bulgarian troops were positioned away from the border in order to not give Serbia cause for attack by defending the border by force. After several incursions, fire was finally exchanged on the . As it happened, the Ottomans did not intervene, and the Serbian Army's advance was stopped after the
Battle of Slivnitsa. The main body of the Bulgarian Army traveled from the Ottoman border in the southeast to the Serbian border in the northwest to defend the capital
Sofia. After the defensive battles at Slivnitsa and
Vidin (the latter's defence was organized by
Atanas Uzunov), Bulgaria began an offensive which took the city of
Pirot. Austria-Hungary then stepped in and threatened to join the war on Serbia's side if the Bulgarian troops did not pull back. No territorial changes were made to either country, but the unification was recognised by the great powers. However, the relationship of trust and friendship between Serbia and Bulgaria, which had been built during their long common fight against Ottoman rule, suffered irreparable damage. == Serbian Army ==