However, after the Battles of Mława and Wizna the German XIX Panzer Corps under General Heinz Guderian broke through Polish lines and sped southward with the aim of flanking Warsaw from the East and cutting Poland in two.

However, the Soviets did not begin their invasion of Poland yet, and had the rapidly advancing German corps stopped, it would give Poles time to regroup and prepare.

Although largely obsolete by contemporary standards, the fortress occupied a strategic position in the Polish lines and its defence could prevent German forces from crossing Polesia into Lesser Poland and Galicia to the south.

The aim of the German XIX Corps was to seize the fortress in order to prevent elements of a divided Independent Operational Group Narew under General Młot-Fijałkowski from retreating southwards and joining the rest of the Polish forces.

Moreover, a large number of newly mobilised reservists started to arrive at the fortress, awaiting forward deployment to their units.

Polish armoured train number 53 (PP53), which made a reconnaissance advance to Wysokie Litewskie, was attacked by a scout patrol from the 10th Panzer Division.

[citation needed] The following day Polish defenders withdrew from the town, but heavy casualties on both sides prevented the German units from continuing the attacks on the fortress.

When reports told Polish General Plisowski that scout elements from the 3rd Panzer Division were seen near the railway station at Żabinka, north of Kobryń, he sent PP55 to prevent his forces from being cut off.

A platoon of five scout tanks left the train near Żabinka and attacked German armoured cars near a bridge on Muchawiec River.

[citation needed] At dawn General Plisowski ordered part of the Polish forces to retreat from the easternmost fortifications and regroup to the other side of the river and southwards.

Some 40 kilometres (25 miles) to the east the Polish improvised "Kobryń" Infantry Division under Colonel Epler was ordered to retreat in tandem with Plisowski.