 Complete the following phrase:  However, Ernst von Mansfeld continued to occupy a portion of the Upper Palatinate and had successfully resisted efforts by Johann Tserclaes, Count of Tilly to dislodge him. Mansfeld crossed into the Rhenish Palatinate in early 1622, and on 21 April 1622, Frederick joined Mansfeld there. Frederick attempted to convince other Protestant princes to reconstitute the Protestant Union, but met with limited success. Fredericks cause was boosted by a 27 April 1622 victory over Tillys forces at the 