At one period of its history Poland was barred from its Baltic sea frontier in the north. In the fourteenth century the invasion of the Teutonic Knights temporarily out off the country from the sea; but apart from this interruption Poland has always had access to the sea to which the drainage of the land naturally led. Under the first members of the Piast dynasty the Poles had control of the Baltic coast.[109] When, in the thirteenth century, the Poles called upon the Knights of the Teutonic Order for assistance in subjugating Prussia, the two parties agreed to equal division of the conquered territory. The successes of the Teutonic Knights, however, emboldened their leaders to claim more land for their share. A state of war ensued between the two former allies until by the treaty of Thorn, in 1466, the Teutonic Knights acknowledged Polish sovereignty. This brought Pomerelia, or Prussian Pomerania, within Polish territory. In 1525 the Prussian districts east of the Vistula became part of the duchy of Albert of Brandenburg and were thus surrounded entirely by Polish territory; but that part of Prussia which extends west of the Vistula remained an integral portion of Poland until 1772. In the fighting which marked the relations between Poland and Turkey in the fourteenth and fifteenth centuries the Poles succeeded in extending their southern frontiers to within a hundred miles of the Black Sea and in carrying their sphere of influence to the sea itself. The occupation of Kaminiec by the Turks was short-lived. In general Poland’s frontier on the side of the ancient Rumanian principalities remained unchanged during the last four centuries of the country’s sovereign existence. In the fifteenth century, Poland was the dominating Slavic state. In 1386 it had been united to Lithuania by Wladislas Jagellon, the first prince of the famous dynasty bearing his name. The country at that time was protected from Turkish attacks by Wallachia, Moldavia and Transylvania. Russia was its rival for the possession of Lithuania; Austria for that of Hungary and Bohemia. Prussia and Livonia were also claimed by Poland from the Order of the Teutonic Knights. The weakness of the country lay in the jealousy of the two peoples of diverse speech from which its ruling body was drawn. The Jagellons were Lithuanian princes. They favored the claims of their countrymen, who preferred the laws of their native land to the Polish legislation which was being forced on them. The Poles likewise had their grievances against the Lithuanians. During the rule of Casimir IV he was frequently taken to task by his countrymen for spending “summer, fall and winter in Lithuania.”