The Union of Polish Patriots has long sought to establish independent Polish units and formations that were to be employed in the titanic struggle that was taking place on the Eastern Front, directly aiding the Soviet armed forces in their drive to defeat Nazi Germany, and eventually liberate Poland itself. The Union's efforts eventually bore fruits, when in early May of 1943 the Soviet government agreed to establish Polish military units on its territory. What would later on be known as the LWP, started out as the Polish 1st Infantry Division "Imienia Tadeusza Kosciuszki". This division was armed and equipped in a manner of a Soviet guards division and began its three-month-long training on 1 June, at its base in Sielce, near Riazan. Many of its soldiers, as was often the case in the subsequent Polish units formed in the Soviet Union and Poland, joined it on a voluntary basis. The division's first battle took place near the village of Lenino, on the Belarussian-Russian republican frontier, between 12 October and the night of 13-14 October, 1943. The battle was short but very fierce, and the division passed its "test of fire" as a fully capable and battle-worthy unit.

On 10 August of 1943, it was decided to expand the Polish military forces in the Soviet Union; therefore, the order to form the 1st Corps of the Polish Armed Forces in the U.S.S.R. was issued; its units took an active part in the liberation of both the Soviet Union (besides Lenino, participation in the liberation of some Soviet territory as part of the 1st Belarussian Front by the artillery, and anti-aircraft defence of Kiev by the anti-aircraft artillery). On 21 July of 1944 the corps was merged with the leftist resistance movement that operated inside Poland and known as the AL (Armia Ludowa - People's Army) to form the LWP (Ludowe Wojsko Polskie - Polish People's Military). On 29 July all the Polish regular military forces on the Eastern Front were redesignated as the 1st Army of the LWP, and thereafter saw heavy action in both Poland and Germany; the 1st Army went on to take an active part in the failed attempts to link up with the Warsaw Uprising, the fighting to preserve the Magnuszew Bridgehead, subsequently in the Warsaw Operation which liberated much of Mazowsze, and then in the breaching of the so-called Pomeranian Wall and, still later, in the Berlin Operation. Four separate units of the 1st Army (the 1st Infantry Division "Imienia Tadeusza Kosciuszki", 2nd Brigade of the Howitzer Artillery, 6th Independent Motorized Pontoon Bridge Battalion, and the 1st Mortar Brigade) took part in the Battle for Berlin (by that is meant the actual fighting for the city of Berlin itself, and not the much more general Berlin Operation which also encompasses many battles fought outside the city). On 2 May of 1945, the victorious Polish troops of the 1st Infantry Division hoisted the Polish flag on the Brandenburg Gate. The rest of the 1st Army took an active part in the Berlin Operation on the fields of Brandenburg, and ended up meeting the Anglo-American troops on the Laba River.

With the liberation of much of Poland, the LWP was able to increase its numerical strength as never before; on 20 August of 1944 an order was issued to form the 2nd Army from the menpower available in Poland; the 2nd Army went on to take an active part, from early 1945, in the liberation of Poland, Germany, and Czechoslovakia. It participated in the heavy fighting to liberate the regions of Milsko and Luzyce, and then was swung southwards to help encircle and eliminate the gigantic German pocket present in Czechia; thus, contributing to what was the most economical Soviet victory against the Nazis of The Great Fatherland War (1941-1945).

On 6 October of 1944 an order was issued to form a 3rd Army; it never passed the formation stage and only part of it saw action against OUN-UPA. On 15 November of the same year it was disbanded, and its existing units were transferred to the two other armies, or became independent units. Meanwhile, embryonic Polish Air Force and Polish Navy started to be formed already on the Soviet soil; the Air Force already saw its first combat action on 23 August of 1944 near Warka, while the Navy (naval infantry) in early 1945 in the Gdansk area.

Near the end of WWII, the LWP numbered around 400 000 troops, which, thanks to the substantial Soviet assistance, formed a modern, experienced, and formidable military force. But the struggle was not over; Polish armed forces would go on to fight against the various Western-backed underground terrorist networks that sprung-up across the liberated Poland, like the post-Nazi German underground (until 1947), OUN-UPA (until 1949), and the Polish reactionary underground (until 1954). Thus, it took many years for the Polish War on Terror to come to an end. Thereafter, the LWP played a prominent role as one of the most important militaries of the Warsaw Pact, playing a major deterrance function against the Western aggression in Europe.