The Mongol invasion, which took place in 1223, destroyed all this young civilisation, and threw Russia into quite new channels. The main cities of South and Middle Russia were laid waste. Kíeff, which had been a populous city and a centre of learning, was reduced to the state of a straggling settlement, and disappeared from history for the next two centuries. Whole populations of large towns were either taken prisoners by the Mongols, or exterminated, if they had offered resistance to the invaders. As if to add to the misfortunes of Russia, the Turks soon followed the Mongols, invading the Balkan peninsula, and by the end of the fifteenth century the two countries from which and through which learning used to come to Russia, namely Servia and Bulgaria, fell under the rule of the Osmanlis. All the life of Russia underwent a deep transformation. Before the invasion the land was covered with independent republics, similar to the mediæval city-republics of Western Europe. Now, a military State, powerfully supported by the Church, began to be slowly built up at Moscow, which conquered, with the aid of the Mongol Khans, the independent principalities that surrounded it. The main effort of the statesmen and the most active men of the Church was now directed towards the building up of a powerful kingdom which should be capable of throwing off the Mongol yoke. State ideals were substituted for those of local autonomy and federation. The Church, in its effort to constitute a Christian nationality, free from all intellectual and moral contact with the abhorred pagan Mongols, became a stern centralised power which pitilessly persecuted everything that was a reminder of a pagan past. It worked hard, at the same time, to establish upon Byzantine ideals the unlimited authority of the Moscow princes. Serfdom was introduced in order to increase the military power of the State. All Independent local life was destroyed. The idea of Moscow becoming a centre for Church and State was powerfully supported by the Church, which preached that Moscow was the heir to Constantinople--“a third Rome,” where the only true Christianity was now to develop. And at a later epoch, when the Mongol yoke had been thrown off, the work of consolidating the Moscow monarchy was continued by the Tsars and the Church, and the struggle was against the intrusion of Western influences, in order to prevent the “Latin” Church from extending its authority over Russia.