Not my composition, but lifted and taken from a source that identified no author, but likely ipn.gov.pl/en
Winston Churchill’s response to the question of the Polish eastern borders
Until the end of 1942, Poland continued to enjoy a relatively strong position among the Allies as a subject to be reckoned with.
A turn for the worse in the quest to rebuild independent Poland took place in 1943 when the Battle of Stalingrad decisively turned the tide on the Eastern Front.
Stalin was now aware that the victorious march of the Red Army westwards was possible, allowing to realize his empire-like plans of conquering at least part of Europe. Moreover, Moscow could now begin to resume spreading the Bolshevik revolution, this time with actual approval from the Western states.
In late 1943, at the Teheran Conference, Stalin received Roosevelt's and Churchill direct permission to annex the eastern half of Poland, who accepted the so-called Curzon Line as the eastern border of the Republic of Poland. In January 1944, the Red Army crossed the eastern border of Poland for the second time during the war.
On 20 January 1944, the government of the Republic of Poland issued a statement that “the Polish nation categorically and unconditionally reject Soviet claims to the eastern territory of the Polish State”. 78 years ago, on 22 February 1944, Winston Churchill gave a speech in the House of Commons. Among the topics was the case of the Polish eastern borders.
“Russia has the right of reassurance against future attacks from the West, and we are going all the way with her to see that she gets it, not only by the might of her arms but by the approval and assent of the United Nations. The liberation of Poland may presently be achieved by the Russian armies after these armies have suffered millions of casualties in breaking the German military machine. I cannot feel that the Russian demand for reassurance about her Western frontiers goes beyond the limits of what is reasonable or just. Marshal Stalin and I also spoke and agreed upon the need for Poland to obtain compensation at the expense of Germany both in the North and in the West”. Despite being one of the victors in WW2, Poland was physically devastated. As a result of the decision of the Great Three, one half of Poland's pre-war territory was turned over to the Soviet Union, and Poland, in turn, gained territories in the West and North at the expense of Germany. As a result, Poland became a country with territory smaller by 20% of its pre-war size.
From 1944 to 1989, Poland was under Communist rule. Despite changes during that period, Poland had no sovereignty though it enjoyed recognition in the international arena.