Owing to unprecedented inundations in the valley of the Danube, it was not till two months after the commencement of the campaign that the Russian army was able to cross that river. It did so at two points, the one in the Dobrudscha, the other at Hirsova. In neither case did it meet with serious opposition. The Turkish army of defence was little inferior in numbers to that of the Russians, but its general, Abdul Kerim, proved to be quite incompetent. He spread his forces in detachments over a front of five hundred miles, and was too late in concentrating them. The Russians, after capturing Nicopolis, the Turkish stronghold on the Danube, advanced into Bulgaria and captured Tirnovo, its ancient capital. Everywhere they were received by the Bulgarians with rapturous demonstrations of delight at the prospect of deliverance from Ottoman rule. General Gourko, with a flying corps, then made a very hazardous but successful march across the Balkans by the Hainköi Pass, and advanced into Bulgaria along the Trudja Valley as far as Eski Zagra. Thence, turning back, he attacked the more important Shipka Pass from the south, and defeated a Turkish force in occupation of it. Meanwhile, early in July, the main Russian army from Tirnovo came in contact at Plevna, twenty miles south of the Danube, with a Turkish army of fifty thousand men under Osman Pasha, who had been sent in relief of Nicopolis, but was too late for the purpose. Plevna was not a fortress. It was a strong natural position, where the Turks entrenched their army behind earthworks and redoubts with great engineering skill, and where they maintained an obstinate and memorable defence for nearly five months, the most striking incident of the campaign of 1877. Three unsuccessful assaults were made by the Russians, assisted by a Roumanian army, in which great losses were incurred. Thereupon, by the advice of General Todleben, the hero of the defence of Sebastopol in the Crimean War, the attempt to take these works at Plevna by assault was given up, and it was subjected to a close investment. The occupation of the Shipka Pass by Gourko prevented the advance of a Turkish army in relief of Plevna, in spite of successive attacks by the Turkish army under Suleiman Pasha. As a result, after five months of heroic resistance, Osman Pasha found himself in great straits for want of food for his army. He determined to make a great effort to break through the lines of the investing army. The sortie failed, and Osman and his whole remaining army of thirty-two thousand men were compelled to surrender on January 9, 1878. This had the effect of releasing the Russian army in front of Plevna. General Gourko and the main part of the Russian army thereupon marched to Sofia. General Skobeleff, in command of another army, determined to force his way across the Balkan range. An army of ninety thousand Turks under another Pasha was stationed at the southern end of the Shipka Pass and barred his way.