The First Balkan War lasted from October 1912 to May 1913 and involved actions of the Balkan League (the Kingdoms of Bulgaria, Serbia, Greece and Montenegro) against the Ottoman Empire. The Balkan states' combined armies overcame the initially numerically inferior (significantly superior by the end of the conflict) and strategically disadvantaged Ottoman armies, achieving rapid success.
The war was a comprehensive and unmitigated disaster for the Ottomans, who lost 83% of their European territories and 69% of their European population. As a result of the war, the League captured and partitioned almost all of the Ottoman Empire's remaining territories in Europe. Ensuing events also led to the creation of an independent Albania, which dissatisfied the Serbs. Bulgaria, meanwhile, was dissatisfied over the division of the spoils in Macedonia and attacked its former allies, Serbia and Greece, on 16 June 1913, which provoked the start of the Second Balkan War.
During the war, many civilians, overwhelmingly Muslim Turks, were either killed or forced to flee their homes. The highly politicized and disorganized units of the Ottoman army were quite incapable of evacuating the civilians in the war zone. This situation left many civilians in the occupied areas defenseless against the invading armies of the Balkan League. Although there are discussions about the exact amount of civilian casualties, when the war ended great changes occurred in the demographic makeup of the Balkan region.
Tensions among the Balkan states over their rival aspirations to the provinces of Ottoman-controlled Rumelia (Eastern Rumelia, Thrace and Macedonia) subsided somewhat after the mid-19th-century intervention by the Great Powers, which aimed to secure both more complete protection for the provinces' Christian majority as well as to maintain the status quo. By 1867, Serbia and Montenegro had both secured their independence, which was confirmed by the Treaty of Berlin (1878). Between 1878 and 1912, the Greece, Bulgaria, and Serbia sponsored insurgent groups in Ottoman Macedonia to fight against the government and each other in a conflict known as the Macedonian Struggle. The question of the viability of Ottoman rule was revived after the Young Turk Revolution in July 1908, which compelled the Ottoman Sultan to restore the suspended constitution of the empire.
Serbia's aspirations to take over Bosnia and Herzegovina were thwarted by the Bosnian crisis, which led to the Austrian annexation of the province in October 1908. The Serbs then directed their war efforts to the south. After the annexation, the Young Turks tried to induce the Muslim population of Bosnia to emigrate to the Ottoman Empire. The Ottoman authorities resettled those who took up the offer in districts of northern Macedonia with few Muslims. The experiment proved to be a catastrophe since the immigrants readily united with the existing population of Albanian Muslims and participated in the series of 1911 Albanian uprisings and the Albanian revolt of 1912. Some Albanian government troops switched sides.
In May 1912, Albanian rebels seeking national autonomy and the re-installment of Sultan Abdul Hamid II to power drove the Young Turkish forces out of Skopje and pressed south towards Manastir (now Bitola), forcing the Young Turks to grant effective autonomy over large regions in June 1912. Serbia, which had been helping to arm the Hamidian and Catholic Albanians rebelling in the Mirditë region; sent secret agents to some of the prominent leaders, taking the revolt as a pretext for war. Serbia, Montenegro, Greece, and Bulgaria had all been in talks about possible offensives against the Ottoman Empire before the 1912 Albanian revolt had broken out, and a formal agreement between Serbia and Montenegro had been signed on 7 March. On 18 October 1912, King Peter I of Serbia issued a declaration, 'To the Serbian People,' which appeared to support Albanians as well as Serbs:
The Turkish governments showed no interest in their duties towards their citizens and turned a deaf ear to all complaints and suggestions. Things got so far out of hand that no one was satisfied with the situation in Turkey in Europe. It also became unbearable for the Serbs, the Greeks, and the Albanians. By the grace of God, I have therefore ordered my brave army to join in the Holy War to free our brethren and to wish for a better future. In Old Serbia, my army will meet not only upon Christian Serbs but also upon Muslim Serbs, who are equally dear to us, and in addition to them, upon Christian and Muslim Albanians with whom our people have shared joy and sorrow for thirteen centuries now. To all of them, we bring freedom, brotherhood and equality.
In a search for allies, Serbia was ready to negotiate a treaty with Bulgaria. The agreement provided that in the event of victory against the Ottomans, Bulgaria would receive all of Macedonia south of the Kriva Palanka–Ohrid line. Bulgaria accepted Serbia's expansion as being to the north of the Shar Mountains (Kosovo). The intervening area was agreed to be "disputed" and would be arbitrated by the Tsar of Russia in the event of a successful war against the Ottoman Empire. During the war, it became apparent that the Albanians did not consider Serbia as a liberator, as had been suggested by King Peter I, and the Serbian forces failed to observe his declaration of amity toward Albanians.
After the successful coup d'état for unification with Eastern Rumelia, Bulgaria began to dream that its national unification would be realized. For that purpose, it developed a large army and was identified as the "Prussia of the Balkans". However, Bulgaria could not win a war alone against the Ottomans.
In Greece, Hellenic Army officers had rebelled in the Goudi coup of August 1909 and secured the appointment of a progressive government under Eleftherios Venizelos, which they hoped would resolve the Crete question in Greece's favour. They also wanted to reverse their defeat in the Greco-Turkish War (1897) by the Ottomans. An emergency military reorganization, led by a French military mission, had been started for that purpose, but its work was interrupted by the outbreak of war in the Balkans. In the discussions that led to Greece joining the Balkan League, Bulgaria refused to commit to any agreement on distributing territorial gains, unlike its deal with Serbia over Macedonia. Bulgaria's diplomatic policy was to push Serbia into one that limited its access to Macedonia while simultaneously refusing any such agreement with Greece. Bulgaria believed that its army could occupy the big part of Aegean Macedonia and the port city of Salonica (Thessaloniki) before the Greeks could do so.
In 1911, Italy had launched an invasion of Tripolitania, now in Libya, which was quickly followed by the occupation of the Dodecanese Islands in the Aegean Sea. The Italians' decisive military victories over the Ottoman Empire and the successful 1912 Albanian revolt encouraged the Balkan states to imagine that they might win a war against the Ottomans. By the spring and summer of 1912, the various Christian Balkan nations had created a network of military alliances, becoming known as the Balkan League.
The Great Powers, most notably France and Austria-Hungary, reacted to the formation of the alliances by trying unsuccessfully to dissuade the Balkan League from going to war. In late September, the League and the Ottoman Empire mobilized their armies. Montenegro was the first to declare war on 25 September (O.S.)/8 October. After issuing an impossible ultimatum to the Ottoman Porte on 13 October, Bulgaria, Serbia and Greece declared war on the Ottomans on 17 October (1912). The declarations of war attracted a large number of war correspondents. An estimated 200 to 300 journalists from around the world covered the war in the Balkans in November 1912.
Due to poor organization, transportation problems and the protracted war with Italy, the Ottoman order of battle only had 12,024 officers, 324,718 other ranks, 47,960 animals, 2,318 artillery pieces and 388 machine guns ready by early October instead of the planned full complement of 750,000 officers and soldiers. A total of 920 officers and 42,607 men of them had been assigned to non-divisional units and services, the remaining 293,206 officers and men were assigned to four armies.