seen as a political risk in Russia, which similarly suppressed its Muslims in Central Asia and Caucasia. Russian progressive opinion became a major domestic factor with its support for liberating Balkan Christians from Ottoman rule and making Bulgaria and Serbia independent. In early 1877, Russia intervened on behalf of Serbian and Russian volunteer forces, leading to the Russo-Turkish War (1877–78). Within one year, Russian troops were nearing Constantinople and the Ottomans surrendered. Russia's diplomats and generals persuaded Alexander II to force the Ottomans to sign the Treaty of San Stefano in March 1878, creating an enlarged, independent Bulgaria that stretched into the southwestern Balkans. At the Congress of Berlin in July 1878, Russia agreed to the creation of a smaller Bulgaria and Eastern Rumelia, as a vassal state and an autonomous principality inside the Ottoman Empire, respectively. As a result, Pan-Slavists were left with a legacy of bitterness against Austria-Hungary and Germany for failing to back Russia. Disappointment at the results of the war stimulated revolutionary tensions, and helped Serbia, Romania, and Montenegro gain independence from, and strengthen themselves against, the Ottomans. Another significant result of the 1877–1878 Russo-Turkish War in Russia's favour was the acquisition from the Ottomans of the provinces of Batum, Ardahan, and Kars in Transcaucasia, which were transformed into the militarily administered regions of Batum Oblast and Kars Oblast. Russia had little difficulty expanding to the south, including conquering Turkestan, until Britain became alarmed when Russia threatened Afghanistan, with the implicit threat to India; and decades of diplomatic maneuvering resulted, called the Great Game. That rivalry between the two empires has been considered to have included far-flung territories such as Outer Mongolia and Tibet. Expansion into the vast stretches of Siberia was slow and expensive, but finally became possible with the building of the Trans-Siberian Railway, 1890 to 1904. This opened up East Asia; and Russian interests focused on Mongolia, Manchuria, and Korea. China was too weak to resist, and was pulled increasingly into the Russian sphere. Russia obtained treaty ports such as Dalian/Port Arthur. In 1894, Alexander III was succeeded by his son, Nicholas II, who was committed to retaining the autocracy that his father had left him. Nicholas II proved as an ineffective ruler, and in the end his dynasty was overthrown by the Russian Revolution. The Industrial Revolution began to show significant influence in Russia, but the country remained rural and poor. By the end of the 19th century, the Russian Empire dominated its territorial extent, covering a surface area of 22,800,000 km², making it become the world's third-largest empire.