Complex sentence: To withstand Saxon overlordship, Wenceslas's father, Vratislaus, had forged an alliance with the Bavarian duke Arnulf the Bad, at that time, a fierce opponent of King Henry; however, it became worthless when Arnulf and Henry reconciled at Regensburg in 921.
Split into simpler sentences: To withstand Saxon overlordship, Wenceslas's father Vratislaus had forged an alliance with the Bavarian duke Arnulf, a fierce opponent of King Henry at that time. The alliance became worthless, however, when Arnulf and Henry reconciled at Regensburg in 921.