On the second day of the Polish offensive, August 16, the division managed to outflank the Bolshevik Mozyr Group by a forced march of over 56 kilometres.

During the battle, the unit formed the core of the Wilno Group and took part in a successful outflanking manoeuvre of the Bolshevik forces centered on the city of Grodno.

Before the outbreak of World War II, the division, commanded by General Wincenty Kowalski, was partially mobilized in March 1939.

As a part of the Wyszków Operational Group it was to shield the northern approaches of Warsaw from the German assault from East Prussia.

After the outbreak of the Invasion of Poland, the division became fully mobilized and on 4 September 1939, it made contact with enemy troops in the forests around Długosiodło.

On 7 September it took part in heavy fighting near Pułtusk, but was outnumbered 3 to 1 and ordered to retreat southwards to defend the Bug River line between Kamieńczyk and Wyszków.

After heavy fighting, the division broke through the third line of German defences in the villages of Lipiny, Debowiec, Wola Wodyńska, and Oleśnica.

It finally broke through the German lines at Jagodno, but also suffered heavy casualties and lost most of its artillery and logistical support.

[5] As an organised military formation, the 1st Legions Infantry Division is also tasked to support the Department for Civil Protection and Crisis Management in case of emergency.