Unfortunately, during the twelfth and thirteenth centuries the realm was rent asunder by partitions and civil wars among its princes, and weakened by the invasions and devastations of the Mongols. Taking advantage of this situation, the Germanic _Drang nach Osten_, which the Poles had hitherto arrested at the cost of much hard fighting, set in with redoubled vigor and unparalleled success. It was at this time that Pomerania and Silesia were lost to Poland,--although in Upper Silesia a large and compact Polish population has for six hundred years successfully resisted that process of more or less violent Germanization to which so many of the western Slavs succumbed. It was at this time also that the Germans, led by the Teutonic Knights, succeeded in planting a colony in Prussia, thus cutting off Poland from the sea and inaugurating that struggle for the mouth of the Vistula which has gone on intermittently ever since. When Poland in the fourteenth century regained her unity and strength, it was now too late to recover most of the territories thus lost: it was rather a question whether the shattered kingdom could even defend what was left to it against the Germanic onrush. Seeking resources and allies for that struggle, Poland turned to the East. The conquest of the principality of Halicz (Eastern Galicia) in 1340 marked the beginning of Polish encroachments upon the Ukrainian nationality. Still more important was the union effected in 1386 between Poland and Lithuania. For the Lithuanian empire, built up with such amazing rapidity during the fourteenth century, included nearly the whole area inhabited by the Ukrainian and White Russian races; it spread from the Baltic to the Black Sea and far beyond the Dnieper. Through the union concluded in 1386 there arose a realm which was to be for several centuries the strongest power in Eastern Europe, and which remained down to the time of the Partitions the second or third largest state on the Continent.