Berling was a co-founder and commander of the First Polish Army, which fought on the Eastern Front of World War II.

After the war, he remained in the military and in 1923 he was promoted to the rank of major, first serving on staff of the 15th Infantry Division of V District Corps Command in Kraków.

[3] After the Sikorski–Mayski agreement of 17 August 1941, Berling was nominated to be chief of staff of the recreated 5th Infantry Division, and later commander of the temporary camp for Polish soldiers in Krasnovodsk.

[3] The sentence was vacated by General Kazimierz Sosnkowski, the Polish commander-in-chief of forces loyal to the London government in exile.

In September 1942 and during the following months, he and Wanda Wasilewska appealed to Joseph Stalin for permission to establish the Polish division.

On 8 April 1943, Berling proposed the establishment of a new Polish army; permission was granted after the break in Soviet-Polish diplomatic relations.

Berling was nominated to be the commander of its first unit, the 1st Tadeusz Kościuszko Infantry Division, and was promoted to general by Stalin.