With the majority of Hideyoshi's forces gone from Omi Province, Katsuie, seeing the situation as an opportunity, accepted a proposal from commanding officer Morimasa SAKUMA to mount an immediate attack on Oiwayama Fort on June 9.
秀吉の軍勢が多く近江から離れたのを好機と見た勝家は部将・佐久間盛政の意見具申もあり、4月19日(旧暦)（6月9日）、盛政に直ちに大岩山砦を攻撃させた。