Turkey, too, was made to feel Nicholas' heavy hand; urged by other powers the sultan submitted to the loss of territory in Asia, which had been in dispute, and permitted the free passage of Russian vessels between the Black Sea and the Mediterranean. (Convention of Akkerman, Oct. 8, 1826.) The czar, after this, took up the Greek question, and entered into an agreement with England and France. In vain did the sultan offer the plea which had been successful with Alexander, that the Greeks "violated the passive obedience owed by subjects to their legitimate sovereigns." Nicholas wanted Turkey for himself, and proposed to leave no stone unturned to secure possession of Constantinople. After the battle of Navarino, on the 20th of October, 1827, where the allied forces destroyed the Turkish fleet. England withdrew, suspicious of Nicholas' schemes; but France and Russia continued the war until by the Peace of Adrianople, the sultan recognized the independence of Greece,--and ceded to Russia four fortresses in Asia and the islands in the delta of the Danube. Russia was thus in possession of the whole southern slope of the Caucasus, besides holding part of its northern front. The czar began war upon the tribes dwelling in the mountains, but found that he had engaged in a very difficult enterprise. A soldier-priest named Schamyl defied the power of Russia for a quarter of a century. It cost Nicholas more in men (p. 210) and money to subdue the liberty-loving mountaineer, than all the wars he waged in Asia. The year 1830, was one of great unrest in Europe. Nicholas was deeply angered when his friend Charles X of France was expelled. The revolution in Paris was the signal for a similar movement in the capital of Poland. Owing to the independent expression of opinion in the Diet, Alexander had adjourned that body indefinitely in 1822. At the same time the liberty of the press was revoked and the police assumed a power in defiance of the law. The Grand Duke Constantine was really a friend of Poland, but he was eccentric and impetuous and often unconsciously gave offense. In 1830, Nicholas came to Warsaw to open the Diet, when its members made demands which he could not grant. Both sides were angry when Nicholas returned to St. Petersburg.