Zubachyov later served during the Winter War with the rank of captain and the post of commander of the battalion of the 44th regiment. The regiment from May 1941 was stationed at Brest Fortress. With the German attack on June 22, due to the fact that the regiment commander Pyotr Gavrilov was cut off with his detachment in the Kobryn fortification, Zubachyov led the defense on the site of the regiment. On June 24 at a meeting of senior officers, he was appointed commander of the consolidated defense group. His deputy was appointed regimental commissar Yefim Fomin. On this council, Zubachyov categorically spoke out against the plans for a breakthrough, believing that the Red Army would soon go on the counteroffensive, and that the garrison's task was to defend the fortress until reinforcements arrived.