The background to the wars lies in the incomplete emergence of nation-states on the European territory of the Ottoman Empire during the second half of the 19th century. Serbia had gained substantial territory during the Russo-Turkish War, 1877–1878, while Greece acquired Thessaly in 1881 (although it lost a small area back to the Ottoman Empire in 1897) and Bulgaria (an autonomous principality since 1878) incorporated the formerly distinct province of Eastern Rumelia (1885). All three countries, as well as Montenegro, sought additional territories within the large Ottoman-ruled region known as Rumelia, comprising Eastern Rumelia, Albania, Macedonia, and Thrace.
Policies of the Great Powers
Throughout the 19th century, the Great Powers shared different aims over the "Eastern Question" and the integrity of the Ottoman Empire. Russia wanted access to the "warm waters" of the Mediterranean from the Black Sea; it pursued a pan-Slavic foreign policy and therefore supported Bulgaria and Serbia. Britain wished to deny Russia access to the "warm waters" and supported the integrity of the Ottoman Empire, although it also supported a limited expansion of Greece as a backup plan in case integrity of the Empire was no longer possible. France wished to strengthen its position in the region, especially in the Levant (today's Lebanon, Syria, the Palestinian territories and Israel).
Habsburg-ruled Austria-Hungary wished for a continuation of the existence of the Ottoman Empire, since both were troubled multinational entities and thus the collapse of the one might weaken the other. The Habsburgs also saw a strong Ottoman presence in the area as a counterweight to the Serbian nationalistic call to their own Serb subjects in Bosnia, Vojvodina and other parts of the empire. Italy, primary aim at the time seems to have been the denial of access to the Adriatic Sea to another major sea power. The German Empire, in turn, under the "Drang nach Osten" policy, aspired to turn the Ottoman Empire into its own de facto colony, and thus supported its integrity.
In the late 19th and early 20th century, Bulgaria and Greece contended for Ottoman Macedonia and Thrace. Ethnic Greeks sought the forced "Hellenization" of ethnic Bulgars, who sought "Bulgarization" of Greeks (Rise of nationalism). Both nations sent armed irregulars into Ottoman territory to protect and assist their ethnic kindred. From 1904, there was low intensity warfare in Macedonia between the Greek and Bulgarian bands and the Ottoman army (the Struggle for Macedonia). After the Young Turk revolution of July 1908, the situation changed drastically.
Young Turk Revolution
The 1908 Young Turk Revolution saw the reinstatement of constitutional monarchy in the Ottoman Empire and the start of the Second Constitutional Era. When the revolt broke out, it was supported by intellectuals, the army, and almost all the ethnic minorities of the Empire, and forced Sultan Abdul Hamid II to re-adopt the long defunct Ottoman constitution of 1876 and parliament. Hopes were raised among the Balkan ethnicities of reforms and autonomy, and elections were held to form a representative, multi-ethnic, Ottoman parliament. However, following the Sultan's attempted counter-coup, the liberal element of the Young Turks was sidelined and the nationalist element became dominant.
At the same time, in October 1908, Austria-Hungary seized the opportunity of the Ottoman political upheaval to annex the de jure Ottoman province of Bosnia and Herzegovina, which it had occupied since 1878 (see Bosnian Crisis). Bulgaria declared independence as it had done in 1878, but this time the independence was internationally recognised. The Greeks of the autonomous Cretan State proclaimed unification with Greece, though the opposition of the Great Powers prevented the latter action from taking practical effect. It has large influence in the consequent world order.
Reaction in the Balkan States
Serbia was frustrated in the north by Austria-Hungary's incorporation of Bosnia. In March 1909, Serbia was forced to accept the annexation and restrain anti-Habsburg agitation by Serbian nationalists. Instead, the Serbian government (PM: Nikola Pašić) looked to formerly Serb territories in the south, notably "Old Serbia" (the Sanjak of Novi Pazar and the province of Kosovo).
On 15 August 1909, the Military League, a group of Greek officers, took action against the government to reform their country's national government and reorganize the army. The Military League found itself unable to create a new political system, until the League summoned the Cretan politician Eleutherios Venizelos to Athens as its political adviser. Venizelos persuaded king George I to revise the constitution and asked the League to disband in favor of a National Assembly. In March 1910, the Military League dissolved itself.
Bulgaria, which had secured Ottoman recognition of her independence in April 1909 and enjoyed the friendship of Russia, also looked to annex districts of Ottoman Thrace and Macedonia. In August 1910, Montenegro followed Bulgaria's precedent by becoming a kingdom.
Bulgarian forces waiting to start their assault on Adrianople
Following Italy's victory in the Italo-Turkish War of 1911–1912, the Young Turks fell from power after a coup. The Balkan countries saw this as an opportunity to attack the Ottoman Empire and fulfill their desires of expansion.
With the initial encouragement of Russian agents, a series of agreements was concluded between Serbia and Bulgaria in March 1912. Military victory against the Ottoman Empire would not be possible while it could bring reinforcements from Asia. The condition of the Ottoman railways of the time was not advanced, so most reinforcements would have to come by sea through the Aegean Sea. Greece was the only Balkan country with a navy powerful enough to deny use of the Aegean to the Ottoman Empire, thus a treaty between Greece and Bulgaria became necessary; it was signed in May 1912.
Montenegro concluded agreements between Serbia and Bulgaria later that year. Bulgaria signed treaties with Serbia to divide the territory of northern Macedonia.
This alliance between Greece, Serbia, Bulgaria, and Montenegro became known as the Balkan League; its existence was undesirable for all the Great Powers. The League was loose at best, though secret liaison officers were exchanged between the Greek and the Serbian army after the war began. Greece delayed the start of the war several times in the summer of 1912, to better prepare her navy, but Montenegro declared war on 8 October (25 September O.S.). Following an ultimatum to the Ottoman Empire, the remaining members of the alliance entered the conflict on 17 October.