The struggle for predominance between Poles and Germans along Poland’s western boundary is fully nine centuries old. In the sixteenth century, Slavonic tribes had become widely distributed between the Oder and Elbe, in the course of westerly expansions which correspond to south and west migrations of Teutonic peoples.[107] Place names bestowed by the early Germans in the district between these two rivers have practically disappeared under the layer of Slavic appellations conferred between the second and fourth centuries.[108] The period between 800 and 1300 witnessed the inception of a slow and powerful Germanic drive directed towards the east. Convents and lay feudal establishments participated in this historical movement. Repeated German aggressions brought about the earliest union of all Polish tribes into one nation at the beginning of the eleventh century. It proved, however, of little avail before the fighting prowess of the Knights of the Teutonic Order, who, by the first half of the thirteenth century, had succeeded in adding all Wend territory to Teutonic dominions. This early and northerly phase of the “Drang nach Osten” brought the Germans to the coast of the Gulf of Finland. Their advance was rendered possible in part by the presence of Tatar hordes menacing southern Poland. Teutonic progress was also facilitated by the defenseless condition which marks an open plain. Between the Oder and the Vistula the slightly undulating lowland is continuous and devoid of barriers to communication which the interposition of uplifted or uninhabitable stretches of territory might have provided. Polish history has been affected both favorably and adversely by this lack of natural bulwarks. The former extension of Polish sovereignty to the shores of the Baltic and Black seas, and to within 50 miles of Berlin and the central plateau of Russia, was a result of easy travel on a plain. This advantage was more than offset by the evident facility with which alien races were able to swarm into the vast featureless expanse forming Polish territory. The dismemberment of the country is in part the result of the inability of the Poles to resort to the protection of a natural fortress, where a prolonged stand against the aggression of foes might have been made. At the end of the tenth century the entire Polish plain acknowledged the rule of Boleslas the Great, a prince of the Piast dynasty. Kiev then paid a yearly tribute to the Polish crown. A period of internal division follows Boleslas’s rule, but in the beginning of the fourteenth century Poland was once more united under the scepter of King Ladislas. From 1386 to 1772, a period of almost four centuries, Polish frontiers remained remarkably stable. Their fluctuations were slight when compared to the changes which occurred in other European countries during the same period.