This union, brought about through the marriage of the Polish princess Yadviga to Yagyello, Grand Prince of Lithuania, and by exceedingly adroit management on the part of the Polish nobles and clergy, opened to the Poles immense regions of country and the way to vast wealth. Before the union their whole land was composed of Great and Little Poland, with Mazovia (see map); after the union two thirds of the best lands of pre-Tartar Russia formed part of the Commonwealth. Since Poland managed to place and maintain itself at the head of affairs, though this roused at all times opposition of varying violence in the other two parts of the Commonwealth, the social ideals and political structure of Poland prevailed in Lithuania and Russia, so far as the upper classes were concerned. In Lithuania, by the terms of the union, all were obliged to become Catholic; in different parts of Russia, which was Orthodox, the people were undisturbed in their religion at first; but after a time the majority of the nobles became Catholic in religion, and Poles in language, name, manners, and ideas. To these was added a large immigration of Polish nobles seeking advancement and wealth. All Russia found itself after a time under control of an upper class which was out of all sympathy with the great mass and majority of the people. During the Yagyellon dynasty, which lasted from 1386 to 1572, the religious question was not so prominent for any save nobles; but ownership of their own land and their own labor was gradually slipping away from the people. During the reign of Sigismund III. (1587-1632), religion was pushed to the foreground, the United Church was brought into Russia; and land and religion, which raise the two greatest problems in a State, the material and the spiritual, were the main objects of thought throughout Russia. Under Vladislav in 1648 the storm burst forth in Southern Russia. There was a popular uprising, the most wide-spread and stubborn in history, during which the Poles lost many battles and gained one great victory, that of Berestechko; the Southern Russians turned to the North, and selected the Tsar Alexai Mihailovich as sovereign. Jan. 8, 1654, there was a great meeting in Pereyaslav,[2] at which Bogdan Hmelnitski, hetman of the Zaporojian army and head of all Southern Russia, after he had consulted with the Cossacks, took his place in the centre of the circle, and in presence of the army, the people, and Buturlin, the envoy of Alexai Mihailovich, said:--