This is a good example of a half truth
"...At the end of the First World War, the boundary between Russia and Poland was settled as being along a line which became known as the Curzon line - Lord Curzon being the British statesman who had proposed it.
This demarcation line was not to the liking of the Poles, who soon went to war against the Soviet Union in order to push their borders further eastward. The Soviet Union counter-attacked and were prepared not only to defend themselves but, against Stalin’s advice, to liberate the whole of Poland. Stalin considered such an aim to be doomed to failure because, he said, Polish nationalism had not yet run its course. The Poles were determined NOT to be liberated so there was no point in trying. Hence the Poles put up fierce resistance to Soviet advances. Ultimately the Soviet Union was forced to retreat and even cede territory to the east of the Curzon line to Poland. The areas in question were Western Byelorussia and the western Ukraine - areas populated overwhelmingly by Byelorussians and Ukrainians respectively rather than by Poles. The whole incident could not but exacerbate the mutual dislike of the Poles and the Russians.
On 1 September 1939, Nazi German invaded Poland. On 17 September, the Soviet Union moved to reoccupy those parts of Poland that lay east of the Curzon line. Having taken over those areas, the Soviet Union set about distributing land to the peasants and bringing about the kind of democratic reforms so popular with the people and so unpopular with the exploiters. During the battle to retake the areas east of the Curzon line, the Soviet Union captured some 10,000 Polish officers, who became prisoners of war. These prisoners were then held in camps in the disputed area and put to work road building, etc.
..
n April 1943, the Hitlerites announced that the Germans had found several mass graves in the Katyn Forest near Smolensk, containing the bodies of thousands of Polish officers allegedly murdered by the Russians.
This announcement was designed to further undermine the co-operation efforts of Poles and Soviets to defeat the Germans. The Russo-Polish alliance was always difficult because the Polish government in exile, based in London, was obviously a government of the exploiting classes. They had to oppose the Germans because of the latter's cynical takeover of their country for lebensraum. The Soviet Union's position was that so long as the Soviet Union could retain the land east of the Curzon line, they had no problem with the re-establishment of a bourgeois government in Poland. But the alliance was already in difficulties because the Polish government in exile, headed by General Sikorski, based in London, would not agree to the return of that land. This is in spite of the fact that in 1941 after Hitler invaded Poland, the Soviet Union and the Polish government in exile had not only established diplomatic relations but had also agreed that the Soviet Union would finance "under the orders of a chief appointed by the Polish government-in-exile but approved by the Soviet government " the formation of a Polish army - this chief being, in the event, the thoroughly anti-Soviet General Anders (a prisoner of the Soviets from 1939). By 25 October 1941 this Army had 41,000 men including 2,630 officers. General Anders, however, eventually refused to fight on the Soviet-German front because of the border dispute between the Soviet Union and Poland, and the Polish army had to be sent elsewhere to fight - i.e., Iran.
Nevertheless, despite the hostility of the Polish government in exile, there was a significant section of Poles resident in the Soviet Union who were not anti-Soviet and did accept the Soviet claim to the territories east of the Curzon line. Many of them were Jewish. These people formed the Union of Polish Patriots which put together the backbone of an alternative Polish government in exile.
The Nazi propaganda relating to the Katyn massacres was designed to make it impossible for the Soviets to have any dealings with the Poles at all. General Sikorski took up the Nazi propaganda with a vengeance, claiming to Churchill that he had a "wealth of evidence". How he had obtained this "evidence" simultaneously with the German announcement of this supposed Soviet atrocity is not clear, although it speaks loudly of secret collaboration between Sikorski and the Nazis. The Germans had made public their allegations on 13 April. On 16 April the Soviet government issued an official communiqué denying "the slanderous fabrications about the alleged mass shootings by Soviet organs in the Smolensk area in the spring of 1940". It added:
"The German statement leaves no doubt about the tragic fate of the former Polish prisoners of war who, in 1941, were engaged in building jobs in areas west of Smolensk and who, together with many Soviet people, fell into the hands of the German hangmen after the withdrawal of Soviet troops".
The Germans had in fabricating their story decided to embellish it with an anti-Semitic twist by claiming to be able to name Soviet officials in charge of the massacre, all of whom had Jewish names. On 19 April Pravda responded:
"Feeling the indignation of the whole of progressive humanity over their massacre of peaceful citizens and particularly of Jews, the Germans are now trying to arouse the anger of gullible people against the Jews. For this reason they have invented a whole collection of 'Jewish commissars' who, they say, took part in the murder of the 10,000 Polish officers. For such experienced fakers it was not difficult to invent a few names of people who never existed - Lev Rybak, Avraam Brodninsky, Chaim Fineberg. No such persons ever existed either in the 'Smolensk section of the OGPU' or in any other department of the NLVD…"
The insistence of Sikorski in endorsing the German propaganda led to the complete breakdown in relations between the London Polish government in exile and the Soviet government - as to which Goebbels commented in his diary:
"This break represents a one-hundred-per-cent victory for German propaganda and especially for me personally … we have been able to convert the Katyn incident into a highly political question. "
At the time the British press condemned Sikorski for his intransigence:
more on this at
http://www.stalinsociety.org.uk/katyn.html
http://en.wikipedia.org/wiki/Curzon_line
So who of you had heard of the Curzon Line before ?
Comrade Stalin