 Complete the following phrase:  In the early 13th century the ties of the Silesian Piasts with the neighbouring Holy Roman Empire grew stronger as several dukes married scions of German nobility. Promoted by the Lower Silesian Duke Henry I the Bearded, from 1230 also regent over Upper Silesia for the minor sons of his late cousin Duke Casimir I of Opole, large parts of the Silesian lands were settled with German immigrants in the course of the Ostsiedlung, establishing numerous cities according to German town law. The plans to re-unify Silesia shattered upon the 