The last Russo-Turkish War (1877-78) was also the most important one. In 1877 Russia and its ally Serbia came to the aid of Bosnia and Herzegovina and Bulgaria in their rebellions against Turkish rule. The Russians attacked through Bulgaria, and after successfully concluding the Siege of Pleven they advanced into Thrace, taking Adrianople (now Edirne, Tur.) in January 1878. In March of that year Russia concluded the Treaty of San Stefano with Turkey. This treaty freed Romania, Serbia, and Montenegro from Turkish rule, gave autonomy to Bosnia and Herzegovina, and created a huge autonomous Bulgaria under Russian protection. Britain and Austria-Hungary, alarmed by the Russian gains contained in the treaty, compelled Russia to accept the Treaty of Berlin (July 1878), whereby Russia's military-political gains from the war were severely restricted.

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Treaty of San Stefano (March 3 [Feb. 19, Old Style], 1878), peace settlement imposed on the Ottoman government by Russia at the conclusion of the Russo-Turkish War of 1877-78; it provided for a new disposition of the European provinces of the Ottoman Empire that would have ended any effective Turkish control over the Balkans if its provisions had not later been modified. Its most important provision established an independent Bulgarian principality, which included most of Macedonia and extended to the Danube and from the Aegean to the Black Sea. The independence of Serbia, Montenegro, and Romania was recognized. The boundaries of Serbia and Montenegro were extended so as to be contiguous, while Romania was compelled to cede southern Bessarabia to Russia, receiving the Dobrudja from Turkey in exchange. Bosnia-Hercegovina was to be autonomous. Parts of Asiatic Turkey were ceded to Russia, and the Ottoman sultan gave guarantees for the security of his Christian subjects.

The treaty was opposed by Austria-Hungary, which disliked encouragement of Slav nationalism, and by the British, who feared the new Bulgarian state would become a Russian satellite and a threat to Istanbul. The treaty was modified by the terms of the Treaty of Berlin signed four months later on July 13.

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Congress of Berlin (June 13-July 13, 1878), diplomatic meeting of the major European powers at which the Treaty of Berlin replaced the Treaty of San Stefano, which had been signed by Russia and Turkey (March 3, 1878) at the conclusion of the Russo-Turkish War of 1877-78. Officially convoked by the Austrian foreign minister, Count Gyula Andrassy, the congress met in Berlin on June 13.

Dominated by the German chancellor Otto von Bismarck, the congress solved an international crisis caused by the San Stefano treaty by revising the peace settlement to satisfy the interests of Great Britain (by denying Russia the means to extend its naval power and by maintaining the Ottoman Empire as a European power) and to satisfy the interests of Austria-Hungary (by allowing it to occupy Bosnia and Herzegovina and thereby increase its influence in the Balkans). In acting so, however, the congress left Russia humiliated by substantially reducing the gains that it had made under the San Stefano treaty. Furthermore, the congress failed to consider adequately the aspirations of the Balkan peoples themselves and, thereby, laid the foundation for future crises in the Balkans.