On November 10, 1918, the regiment's first battalion, under Major Julian Stachiewicz, left Kraków for Przemyśl, to be immediately involved in heavy fighting against the Ukrainians.

Fighting around Lwów continued until early spring 1919, and the regiment remained in the area of the city until March/April 1919, when it was transferred to Ostrów Mazowiecka, where it joined the 1st Legions Infantry Division, and was sent to the Lithuanian-Belarusian front.

On April 17, 1919, the 5th Legions Infantry Regiment occupied the rail junction of Lida, and continued fighting near Vilnius until late summer.

The regiment kept the traditions of the Polish Legions, with special cult of Józef Piłsudski, whose birthday (March 19) was celebrated by the soldiers and officers.

Since the 5th Legions Infantry Regiment of Józef Piłsudski was regarded as an elite unit, it case of war it was mobilized in the first wave.

On March 22, 1939, regimental officers, who were taking part in winter exercises near Vilnius, were recalled back to the city by General Wincenty Kowalski, commandant of the 1st Legions Infantry Division.

On August 15, the Armed Forces Day, after a field service on Marshal Pilsudski Square in Vilnius, a military parade took place, with President Ignacy Mościcki present.

The regiment completed the mobilization on August 26, and on the next day's evening, it marched in silence before Pilsudski's tomb at Rasos Cemetery.

Together with whole 1st Legions Infantry Division, the concentration area was located between the Bug and the Narew, in Puszcza Biała's forests.

The German attack began in the morning of September 9, and was repelled by the Poles, but soon afterwards, Polish Army Headquarters ordered another retreat, towards Biała Podlaska and southeastern corner of the country.

Finally, on September 18 near Chełm, all survivors of the 1st and 3rd Legions Infantry Division were merged into one unit, totalling 6,000 soldiers.

[citation needed] The flag featured silver Polish Eagle, with the inscription HONOUR AND FATHERLAND and names and dates of major battles (KRZYWOPLOTY 15.