In 1585, when Yoshishige SATAKE (the 18th family head) and Kunitsuna UTSUNOMIYA attacked Sukeharu NASU and Yoshitake MIBU, Ujimasa began a full-blown invasion of Shimotsuke Province by building an alliance with the Nasu clan and put the southern half of the Province under its control.
天正13年（1585年）、佐竹義重(十八代当主)・宇都宮国綱らが那須資晴・壬生義雄らを攻めると、氏政は那須氏らと手を結んで本格的に下野侵攻を開始し、下野の南半分を支配下に置いた。