The bulk of the Polish Armed Forces in United Kingdom has been formed from Polish citizens who were either residing there before the war or have been transported to British territories from France after the surrender of the later country in 1940. Polish units in British territories were also formed of soldiers released from the Soviet camps and prisons who were transferred to Iran and then to North Africa.

It is true that a small part of the armed forces constituted of people travelling from the occupied Poland but these were relatively rare cases due to the measures taken by the occupying forces made leaving the Polish territory definitely dangerous, while a significant part of the countries located on most logical routes from Poland to Great Britain were either Axis members, occupied by the latter, more or less allied with them or at least were neutral and reluctant to facilitate transfer of potential combatants. Either way, these circumstances did not help transfer of the entire units of even larger numbers of Polish nationals, including soldiers.

Having realized that in the case of conflict between Poland and Germany, the former are very unlikely to successfully defend without external help, Polish government sought the means to form the Polish units beyond the state borders in 1939. These preliminary talks were hastened by the outbreak of war in Europe, and on 9th September, Polish and French government agreed to form a single Polish division with the possibility of expanding Polish forces by further units. Władysław Sikorski, Prime Minister of Poland proposed even to form two infantry corps and one armoured division. The formation of Polish armed forces in France has been formalized only on 4th January 1940 with an act that sanctioned the creation of Polish units with the main training centre located near Coëtquidan. These units were to have their own command that was separate but was reporting to the commanders of the French army. The main source of manpower were assumed to be people residing outside of Polish borders, including Polish residents of Western Europe (mainly France, United Kingdom and Netherlands), with the possibility of inclusion of the people escaping the occupation or internment and, if possible, also Polish citizens residing in Americas, most notably in the United States and Canada. Although the latter were reluctant to join the forces for variety of reasons, they continued to significantly support the Polish war effort throughout the entire conflict. The number of Polish residents of the Western Europe and people slowly trickling from the internment places (most notably Romania, Hungary, Lithuania and Latvia) allowed the formation of two infantry divisions (1st Grenadiers and 2nd Rifles), one armoured brigade (10th Armoured Cavalry) and two independent infantry brigades (Carpathian Rifles and Podhale Rifles) with further two infantry divisions not achieving full strength before the surrender of France in 1940. In addition, almost 7000 Polish nationals joined the air forces, out of which roughly 560 were trained pilots. They formed several fighter squadrons, a bomber and recon squadrons, with part of the Polish airmen recruited into French units. All aforementioned Polish units formed in France thus amounted to roughly 45.000 soldiers.

All these units, save for the Independent Carpathian Rifles Brigade, took part in the defence of France, but between combat losses and disruption, only 27.000 soldiers (including 6500 in the air forces and 1500 in the navy) were finally extracted to Great Britain with part of the soldiers stranded in France arriving later through the enemy lines or joining the French Resistance. Polish soldiers extracted from France to Great Britain were transferred to the temporary camps in Biggar, Crawford and Douglas, where they were reformed into new units. In July 1940, 1st and 2nd Rifle Brigades as well as the 1st Tank regiment were created. They were joined later that year by three new Rifle Brigades. On 28th September 1940 these units and few others that were created in the meantime, were included into the First Polish Corps. The specific units were often reformed and renamed in accordance with the requirements of the British Army, but in general, Polish soldiers were spending most of their time before the 1944 training and defending the Great Britain from potential German invasion, after which they took part in operation Overlord and liberation of France, with the exception of airmen, who, with the fighter pilots organized in 13 fighter squadrons and bomber crewmen recruited into Royal Air Force took part in the Battle of Britain and other air operation since mid-1940. In the early 1944, Polish forces were bolstered with a substantial number of the Polish soldiers who were pressed into service in the German army and were subsequently taken prisoner, primarily in the African and Italian campaigns (Germans preferred to use soldiers from occupied territories in Africa to discourage desertion).

Independent Carpathian Rifles Brigade has been formed not in France proper, as it was originally a part of the French Levant Army in French territories held in what is now Syria, with the main camp located in Homs (its members were primarily recruited from the soldiers interned in Romania and Hungary who managed to arrive to Middle East by sea via Black Sea and Bosporus. Although the commander of the Levant Army, Gen. Eugene Mittelhausser supported Vichy Government, he allowed The Carpathian Rifles to leave Syria. The unit has been ordered by the Polish Chief of Command to join the British forces and thus was relocated to Palestine and later to Alexandria, from where they were dislocated to Mersa Matruh where they served in the support roles (Italy and Poland were officially not at war) and later to Tobruk, Al-Ghazala and Cirene, where they fought German troops.