The Serbo-Bulgarian War or the Serbian–Bulgarian War (Bulgarian: Сръбско-българска война, Srăbsko-bălgarska voyna, Serbian: Српско-бугарски рат, Srpsko-bugarski rat), a war between the Kingdom of Serbia and the Principality of Bulgaria, erupted on 14 November [O.S. 2 November] 1885 and lasted until 28 November [O.S. 16 November] 1885. Despite Bulgaria's status as a vassal of the Ottoman Empire, the Ottomans did not intervene in the war. Serbia initiated the fighting but suffered a decisive defeat. Austria-Hungary demanded that Bulgaria stop its invasion, and a truce resulted.
The final peace was signed on 3 March [O.S. 19 February] 1886 in Bucharest. The existing boundaries did not change. As a result of the war, European powers acknowledged the act of Unification of Bulgaria which had happened on 18 September [O.S. 6 September] 1885.
On 18 September [O.S. 6 September] 1885, 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 14 November [O.S. 2 November] 1885. 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 13th of November [O.S. 1 November] 1885.
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.
The Serbian army's infantry weaponry stood up to the most modern standards of the time (Mauser-Milovanović single fire rifles with excellent ballistic characteristics). However, the artillery was ill-equipped, still using muzzle-loading cannons of the La Hitte system. Breech-loading cannons of the De Bange system had been ordered and paid for, but did not arrive in Serbia until 1886. The total number of Serbian armed forces expected to take part in the military operation was about 60,000. King Milan I divided his force into two armies, the Nishava and Timok armies.
The former undertook the main objective, to overcome the Bulgarian defences along the west border, to conquer Sofia and advance towards the Ihtiman heights. It was there that the army was supposed to encounter and crush the Bulgarian forces coming from the southeast. Serbia's main advantages on paper were the better small arms and the highly educated commanders and soldiers, who had gained a great deal of experience from the last two wars against the Ottoman Empire.
However, internal Serbian problems, supplemented by King Milan's conduct of the war, nullified most of these advantages:
To claim all the glory for the victory he considered imminent, King Milan did not call upon the most famous commanders of the previous wars (Gen. Jovan Belimarković, Gen. Đura Horvatović and Gen. Milojko Lešjanin) to command the army. Instead, he took the position of army commander himself and gave most of the divisional commands to officers chosen primarily for their loyalty and not war records like Petar Topalović of the Morava division, who had previously commanded the troops suppressing the Timok Rebellion, which was militarily poorly organised.
Furthermore, underestimating the Bulgarian military strength and fearing mutinies for conducting such an unpopular war and having indeed experienced the Timok Rebellion two years earlier, he ordered the mobilisation of only the first class of infantry (recruits younger than 30 years), which meant mobilising only about half of the available Serbian manpower. In doing so, he deprived the Serbian Army of its veterans of the previous wars against the Ottoman Empire.
The modern rifles, despite being among the best in Europe at the time, still had issues of their own: they were introduced only two years before the outbreak of the war, and as such, many of the soldiers were not well-trained in their use. More importantly, the theoretical capabilities of the rifle often misled the Serbian officers, who still lacked experience with it, into ordering volleys from distances of half a mile or more, which wasted precious ammunition for negligible results.
Furthermore, the quantity of ammunition purchased was based on the consumption of bullets by the previous much older and slower-firing rifles. The situation was made worse still by the contemporary Serbian tactics, which emphasised firepower and downplayed hand-to-hand fighting, which contributed to heavy casualties in the fight for Neškov Vis to defend Pirot.
Bulgaria was forced to meet the Serbian threat with two serious disadvantages. When the unification had been declared, Russia had withdrawn its military officers, who had until then commanded all larger units of Bulgaria's young army. The remaining Bulgarian officers had lower ranks and no experience in commanding units larger than platoons (causing the conflict to be dubbed "The War of the Captains"). Also, since the Bulgarian government had expected an attack from the Ottoman Empire, the main forces of the Bulgarian Army were along the southeastern border. Their redeployment across Bulgaria would take at least five to six days.