During the Battle of the Border (particularly the battle of Mława on September 1–3) the Army was pushed back by the German Third Army, towards the Narew and Bug rivers. It received reinforcements in the form of the Operational Group Wyszków under General Wincenty Kowalski, but they were not sufficient to stop it from having to fall back. Eventually, around September 10, the Army abandoned its positions near Warsaw; some of the units (primarily the 20th Infantry Division) remained in the Warsaw area, joining the Warszawa Army taking part in the final defense of Warsaw; others were pushed to the south east (Romanian Bridgehead) and took part in the Battle of Tomaszów Lubelski (September 21–26), after which most of them were forced to surrender.