Bismarck, drawing its fire for an hour while the Royal Navy caught up in time to destroy the German warship. The Poles formed the fourth-largest Allied armed force after the Soviets, the Americans, and the combined troops of the British Empire. They were the largest group of non-British personnel in the RAF during the Battle of Britain, and the 303 Polish Squadron was the most successful RAF unit in the Battle of Britain. Special Operations Executive had a large section of covert, elite Polish troops who cooperated closely with the Polish underground army. By July 1945 there were 228,000 troops of the Polish Armed Forces in the West serving under the British. Many of these men and women came from the Kresy region (eastern Poland), including from the major cities of Lwów (now Lviv, Ukraine) and Wilno (now Vilnius, Lithuania). They had been deported by the Soviets from the Kresy to the gulags when Nazi Germany and the Soviet Union occupied Poland in 1939 under the Nazi-Soviet Pact. Two years later, when Churchill and Joseph Stalin formed an alliance against Adolf Hitler, the mostly "Kresy Poles" were released from the Gulags in Siberia to form "Anders' Army" and were made to walk via Kazakhstan, Uzbekistan and Turkmenistan, where thousands perished on the way, to Iran. There the Polish II Corps came into being under British command. They fought in the battles of Monte Cassino, the Falaise Gap, Arnhem, Tobruk, and in the liberation of many European cities, including Bologna and Breda. The Polish troops who contributed to the Allied defeat of the Germans in North Africa and Italy, had expected to be able to return at war's end to their Kresy (eastern Polish) homeland in an independent and democratic Poland. But at Yalta, Roosevelt and Churchill acquiesced in Stalin's Soviet Union annexation of the Kresy lands (roughly half of pre-war Poland's landmass), in accordance with the provisions of the 1939 Nazi-Soviet Pact. This entailed massive postwar Polish population deportations to western so-called "Recovered Territories" assigned from Germany to Poland. The great majority of Polish soldiers, sailors, and airmen in the West would never return to their homeland. In apparent reaction to British acquiescence in Poland's postwar future, thirty officers and men of the Polish II Corps committed suicide. Churchill explained the government's actions in a three-day Parliamentary debate, begun on 27 February 1945, which ended in a vote of confidence.