The fall of Kiev and Ukraine's subsequent loss of autonomous statehood in the fourteenth century can only be ascribed to the old system of military conquest. The affairs of eastern Ukraine became confused and decadent through the constant Mongol pressure which began in the thirteenth century. One hundred years later, part of western Ukraine also, weakened by frequent Tatar invasions, fell a prey to Poland, to whom she was a tempting prize because of her rich soil. The Polish conquest of Ukraine started in 1340 and, after thirty-five years of the bitterest warfare, the Poles succeeded in annexing an area of land approximately coextensive with the present provinces of Kholm and Eastern Galicia. This they never succeeded in assimilating, in spite of the most tremendous efforts. Simultaneously Volhynia and other northern Ukrainian territories became confederated with Lithuania in order to gain protection against the Tatars. The marriage of the Lithuanian king to the Queen of Poland and the union of the two realms drew these Ukrainian lands also in 1386 into an informal union with the Polish empire which, in 1569, in spite of Ukrainian protests, was made definite, and lasted until 1648. In that year the whole Ukrainian people rose, under the leadership of Bohdan Khmelnitsky, and put an end to this union, which was incompatible with their interests and with their type of civilization. Then, anticipating further Polish efforts to destroy the newly won independence of Ukraine, and menaced by other foes, particularly the Turks, then the strongest military power in eastern Europe, the Ukrainians concluded an agreement of confederation with the Czar of Muscovy in 1654. It is interesting to recall that Khmelnitsky was expressly advised against this step by Oliver Cromwell, who declared that the Czar would never permanently recognize a free people. The most important clauses in the treaty of 1654 guaranteed a freely chosen supreme head for the Ukrainian state, called a "hetman"; the right to engage in diplomatic relations with other states, except Poland and Turkey, when the cognizance of the Czar was necessary; free trade with all foreign nations; the complete independence of the judicial system; the right to choose a leader for the army, over whom the "hetman" had supreme control; and, lastly, the independence of the Ukrainian Church.