Although the training of the new recruits was not finished, the extremely difficult situation on all fronts of the Polish-Bolshevik War forced the General Staff to transfer it to the front. The brigade was transferred to the area of the Modlin Fortress and on August 13 joined the 5th Army under general Władysław Sikorski. The Siberian Brigade became a core of the Polish defence lines in the area and managed to hold out all assaults on the fortress organised by the Red Army. After the Battle of Warsaw the brigade started a pursuit after the fleeing enemy forces and broke through the enemy front in the battles of Borków, Zawady and Joniec. Between August 22 and August 24, 1920, the Brigade fought heavy battles against the Red 4th Army and 3rd Cavalry Corps under Gay Dimitrievich Gay. Its elements took part in the later Battle of the Niemen and several skirmishes with Lithuanian forces occupying the region of Suwałki.