The Balkan Wars were two conflicts that took place in the Balkan states in 1912 and 1913. In the First Balkan War, the four Balkan states of Greece, Serbia, Montenegro and Bulgaria declared war upon the Ottoman Empire and defeated it, in the process stripping the Ottomans of their European provinces, leaving only Eastern Thrace under Ottoman control. In the Second Balkan War, Bulgaria fought against the other four combatants of the first war. It also faced an attack from Romania from the north. The Ottoman Empire lost the bulk of its territory in Europe. Although not involved as a combatant, Austria-Hungary became relatively weaker as a much enlarged Serbia pushed for union of the South Slavic peoples. The war set the stage for the July crisis of 1914 and as a prelude to the First World War.^{[page needed]}

By the early 20th century, Bulgaria, Greece, Montenegro and Serbia had achieved independence from the Ottoman Empire, but large elements of their ethnic populations remained under Ottoman rule. Over the course of the Macedonian Struggle these states fought for influence between themselves and the Ottoman government within Ottoman Macedonia, during which their governments came under the control of nationalists. In 1912, these countries united to formed the Balkan League. The First Balkan War began on 8 October 1912, when the League member states attacked the Ottoman Empire, and ended eight months later with the signing on 30 May 1913 of the Treaty of London negotiated together with the Great Powers. The Great Powers—particularly Italy and Austria-Hungary—included independence for Albania in the Treaty. The Second Balkan War began on 16 June 1913, when Bulgaria, dissatisfied with its allotment of territory from Macedonia, attacked its former Balkan League allies. The combined forces of the Serbian and Greek armies, with their superior numbers repelled the Bulgarian offensive and counter-attacked by invading Bulgaria from the west and the south. Romania, having taken no part in the first conflict, had intact armies to strike with and invaded Bulgaria from the north in violation of a peace treaty between the two states. The Ottoman Empire also attacked Bulgaria and advanced in Thrace, regaining Adrianople. In the resulting Treaty of Bucharest, Bulgaria managed to retain most of the territories it had gained in the First Balkan War. However, it was forced to cede the ex-Ottoman south part of Dobruja province to Romania.

The Balkan Wars were marked by ethnic cleansing, with all parties being responsible for grave atrocities against civilians, and inspired later atrocities including war crimes during the 1990s Yugoslav Wars.

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.

The First Balkan War had some main causes, which included:

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; so, 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 Ottoman Empire was not possible. France wished to strengthen its position in the region, especially in the Levant (today's Lebanon, Syria, 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's 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.

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. It forced Sultan Abdul Hamid II to re-adopt the defunct Ottoman constitution of 1876 and parliament. Hopes were raised among the Balkan ethnicities of reforms and autonomy. Elections were held to form a representative, multi-ethnic, Ottoman parliament. However, following the Sultan's failed counter-coup of 1909, the liberal element of the Young Turks was sidelined and the nationalist element became dominant.