Tensions among the Balkan states over their rival aspirations to the provinces of Ottoman-controlled Rumelia, namely Eastern Rumelia, Thrace and Macedonia, subsided somewhat following intervention by the Great Powers in the mid-19th century, aimed at securing both more complete protection for the provinces' Christian majority and protection of the status quo. By 1867, Serbia and Montenegro had both secured independence, which was confirmed by the Treaty of Berlin (1878). The question of the viability of Ottoman rule was revived after the Young Turk Revolution of July 1908, which compelled the Sultan to restore the suspended Ottoman constitution.
Serbia's aspirations to take over Bosnia and Herzegovina were thwarted by the Bosnian crisis and the Austrian annexation of the province in October 1908. The Serbs directed their expansionism to the south. Following the annexation, the Young Turks tried to induce the Muslim population of Bosnia to emigrate to the Ottoman Empire. Those who took up the offer were re-settled by the Ottoman authorities in districts of northern Macedonia where there were few Muslims. The experiment proved to be a catastrophe for the Empire since the immigrants readily united with the existing population of Albanian Muslims. They participated in the series of Albanian uprisings before and during the spring Albanian Revolt of 1912. Some Albanian government troops switched sides.
In May 1912, the Albanian Hamidian revolutionaries, who wanted to reinstall Sultan Abdulhamit II to power, drove the Young Turkish forces out of Skopje and pressed south towards Manastir (present day Bitola), forcing the Young Turks to grant effective autonomy over large regions in June 1912. Serbia, which had helped arm the Albanian Catholic and Hamidian rebels and sent secret agents to some of the prominent leaders, took 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 Albanian revolt of 1912 broke out; a formal agreement between Serbia and Montenegro had been signed on 7 March. On 18 October 1912, 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 became unbearable for the Serbs, the Greeks and for the Albanians, too. By the grace of God, I have therefore ordered my brave army to join in the Holy War to free our brethren and to ensure 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. Serbia's expansion was accepted by Bulgaria as being to the north of the Shar Mountains (i.e. Kosovo). The intervening area was agreed to be "disputed"; it would be arbitrated by the Tsar of Russia in the event of a successful war against the Ottoman Empire. During the course of the war, it became apparent that the Albanians did not consider Serbia as a liberator, as suggested by King Peter I, nor did the Serbian forces 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 identified as the "Prussia of the Balkans." But 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 of 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 Greece to join the Balkan League, Bulgaria refused to commit to any agreement on the distribution of territorial gains, unlike its deal with Serbia over Macedonia. Bulgaria's diplomatic policy was to push Serbia into an agreement limiting its access to Macedonia, while at the same time refusing any such agreement with Greece. Bulgaria believed that its army would be able to occupy the larger part of Aegean Macedonia and the important port city of Salonica (Thessaloniki) before the Greeks.
In 1911, Italy had launched an invasion of Tripolitania in present-day 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 encouraged the Balkan states to imagine 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 which became known as the Balkan League.
The Great Powers, most notably France and Austria-Hungary, reacted to the formation of these alliances by trying to dissuade the League from going to war, but failed. In late September, both 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 Porte on 13 October, Bulgaria, Serbia and Greece declared war on the Empire on 17 October. The declarations of war attracted a large number of war correspondents. An estimated 200–300 journalists from around the world covered the war in the Balkans in November 1912.