After Poland's defeat in September-October 1939, the Polish government-in-exile quickly organized in France a new fighting force originally of about 80,000 men. Their units were subordinate to the French Army. In early 1940 a Polish Independent Highland Brigade took part in the Battles of Narvik in Norway. A Polish Independent Carpathian Brigade was formed in the French Mandate of Syria, to which many Polish troops had escaped from Poland. The Polish Air Force in France comprised 86 aircraft in four squadrons, one and a half of the squadrons being fully operational while the rest were in various stages of training. Two Polish divisions (First Grenadier Division, and Second Infantry Fusiliers Division) took part in the defence of France, while a Polish motorized brigade and two infantry divisions were being formed.

At the capitulation of France, General Władysław Sikorski (the Polish commander-in-chief and prime minister) was able to evacuate many Polish troops—probably over 20,000—to the United Kingdom.

The Polish Navy had been the first to regroup off the shores of the United Kingdom. Polish ships and sailors had been sent to Britain in mid-1939 by General Sikorski, and a Polish-British Naval agreement was signed in November of the same year.[5] Under this agreement, the Polish sailors were allowed to don Polish uniforms and commanding officers were Polish, however the ships used were of British make.[5] By 1940, the sailors had already impressed Winston Churchill, who remarked that he had "rarely seen a finer body of men".

After being evacuated after the defeat of France, Polish fliers had an important role in the Battle of Britain. At first, the Polish pilots were overlooked, despite their numbers being high (close to 8,500 by mid-1940. Despite having flown for years, most of them were posted either to RAF bomber squadrons or the RAF Volunteer Reserve. This was due to lack of understanding in the face of Polish defeat by the Germans, as well as language barriers and British commanders' opinion of Polish attitudes.

On 11 June 1940, the Polish Government in Exile finally signed an agreement with the British Government to form a Polish Air Force in the UK, and in July 1940 the RAF announced that it would form two Polish fighter squadrons: 302 "Poznański" Squadron and 303 "Kościuszko" Squadron. The squadrons were composed of Polish pilots and ground crews, although their flight commanders and commanding officers were British. Once given the opportunity to fly, it did not take long for their British counterparts to appreciate the tenacity of the Poles. Even Air Officer Commanding Hugh Dowding, who had been one of the first to voice his doubt of the Poles, said:I must confess that I had been a little doubtful of the effect which their experience in their own countries and in France might have had upon the Polish and Czech pilots, but my doubts were laid to rest, because all three squadrons swung into the fight with a dash and enthusiasm which is beyond praise. They were inspired by a burning hatred for the Germans which made them very deadly opponents.

Dowding later stated further that "had it not been for the magnificent [work of] the Polish squadrons and their unsurpassed gallantry, I hesitate to say that the outcome of the Battle would have been the same."

As for ground troops, some Polish ground units regrouped in southern Scotland. These units, as Polish I Corps, comprised the 1st Independent Rifle Brigade, the 10th Motorised Cavalry Brigade (as infantry) and cadre brigades (largely manned by surplus officers at battalion strength) and took over responsibility in October 1940 for the defence of the counties of Fife and Angus; this included reinforcing coastal defences that had already been started. I Corps was under the direct command of Scottish Command of the British Army. While in this area, the Corps was reorganised and expanded.

The opportunity to form another Polish army came in 1941, following an agreement between the Polish government in exile and Joseph Stalin, the Soviets releasing Polish soldiers, civilians and citizens from imprisonment. From these a 75,000-strong army was formed in the Soviet Union under General Władysław Anders (Anders' Army). This army, successively gathered in Bouzoulouk, Samarkand, was later ferried from Krasnovodsk across the Caspian Sea to the Middle East (Iran) where Polish II Corps was formed.

By March 1944, the Polish Armed Forces in the West, fighting under British command, numbered 165,000 at the end of that year, including about 20,000 personnel in the Polish Air Force and 3,000 in the Polish Navy. By the end of the Second World War, they were 195,000 strong, and by July 1945 had increased to 228,000, most of the newcomers being released prisoners-of-war and ex-labor camp inmates.

The Polish Armed Forces in the West fought in most Allied operations against Nazi Germany in the Mediterranean and Middle East and European theatres: the North African Campaign, the Italian Campaign (with Battle of Monte Cassino being one of the most notable), the Western European Campaign (from Dieppe Raid and D-Day through Battle of Normandy and latter operations, especially Operation Market Garden)

After the German Instrument of Surrender, 1945, Polish troops took part in occupation duties in the Western Allied Occupation Zones in Germany. A Polish town was created: it was first named Lwow, then Maczkow.

Polish troops were incorporated into the 1945 top secret contingency plan, Operation Unthinkable, the hypothetical attack on the Soviet Union that would have led to an independent Poland.