Another line of defence, along the Bug river, also was impossible to hold, as units of the Northern Front were scattered after heavy fighting against the Germans.
The Prusy Army, destroyed near Piotrków Trybunalski and Radom, was slowly recreated in the area of Chełm.
The Northern Front for the first time got in contact with the enemy in the night of September 16/17, 1939, when Wehrmacht units, advancing from Brzesc nad Bugiem, reached the town of Włodawa.
Very few soldiers and officers managed to flee southwards, facing not only the Wehrmacht, but also the Red Army, which had invaded eastern Poland on September 17.
These forces capitulated on September 26, after realizing that it was impossible to cross the heavily guarded San river.