The Second World War is usually dated from the German invasion of Poland, 1 September 1939. Both Britain and France had given guarantees to protect Poland from attack. Especially Stanisław Mikołajczyk insisted Britain had to keep its promise, and therefore he was forced to resign from the government-in-exile in November 1944.
In 1945, Poland's borders were redrawn, following the decision taken at the Teheran Conference of 1943 at the insistence of the Soviet Union. The eastern Polish territories which the Soviet Union had occupied in 1939 (minus the Bialystok region) were permanently annexed, and most of their Polish inhabitants expelled. Today these territories are part of Belarus, Ukraine and Lithuania.
Poland received former German territory east of the Oder-Neisse line in turn, consisting of the southern two thirds of East Prussia and most of Pomerania, Neumark (East Brandenburg), and Silesia. The German population was expelled before these "recovered territories" were repopulated with Poles from central Poland and those expelled from the eastern regions.
Polish resistance fighters were incarcerated or deported to Siberia by Stalin, in line with decisions forced upon Churchill and Roosevelt.
The fact that Western leaders tried to force Polish leaders to accept the conditions of Stalin is a matter of continuing resentment for some Poles even today. Some view it as a "betrayal" of Poland by the Western allies (which can be seen as part of a larger "betrayal" to allow it to fall entirely into the Soviet sphere of influence anyway). Moreover, it was used by ruling communists to underline anti-Western sentiments. It was easy to argue that Poland wasn't too important to the West, since its leaders sacrificed Poland's borders, legal government, and free elections.
Defenders of the actions taken by the Western allies maintain that realpolitik made it impossible to do anything else, and that they were in no shape to start a war with the Soviet Union over the subjugation of Poland and other Eastern-European countries immediately after the end of World War II. Arthur Bliss Lane, the US Ambassador to Poland at the time, claimed that some actions of the Secretary of State were a result of ignorance rather than realpolitik.
The latest discussion indicates that the real problem was that Western politicians had promised Stalin that they would settle the issue of borders with the Poles, but failed to do so. The Polish Prime Minister, expecting a serious debate on the borders, faced Stalin, who thought that this problem had already been solved—in his favour. The result was the failure of the Warsaw Uprising, and 200,000 civilian victims.