When the Germans invaded on September 1, not all of the planned fortifications had been completed; some of the Army's units (such as the
Mazowiecka Cavalry Brigade) had arrived in their designated areas only by 30 August (and its area had only 3 out of 40 planned fortifications completed). 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. ==Organization==