Many Jews moved from Germany, England and France to Poland after the First Crusade in the 12th century. They chose Poland because the country had a high religious tolerance and social autonomy.
The Polish kings encouraged thousands of German Jews to move to their country after the great Mongol invasions in the 13th century. They did so because many of the Jews had money and were well educated, which meant they could contribute to the rebuilding of the country ruined by the Mongols. Moreover, thousands of Jews moved to Poland after the Black Death plague of the 14th century to escape persecution as Christians blamed them for the disease.