[The Soviet Union severed diplomatic relations with Poland on 25 April. The dispute had its origin in the question of Poland's eastern frontier: Poland insisted on the border established in 1921 by the Treaty of Riga after the Polish-Soviet war of 1919-20, which had remained the boundary until 1939, while the Soviet Union claimed the border of 1939, which approximated the Curzon Line drawn twenty years earlier and which was closer to the ethnographic divide. This disagreement was still unresolved in April 1943 when the Germans released details of the discovery in Katyn Forest, near Smolensk, of the remains of 4600 Polish officers who the Germans alleged had been murdered by Soviet authorities at some stage after the Soviet occupation of eastern Poland in 1939. The Polish Govt immediately asked the International Red Cross to investigate the matter, whereupon the Soviet Govt severed relations on the grounds that the Polish action gave credence to the German allegations and was an action unbecoming an ally.]
Soviet-Polish dispute. You will have seen Dominions Office background telegram April 23rd (D.238)  and a further telegram is being sent you today. 
My own personal impression of this most regrettable happening is that whatever the merits of the case may be-and in this connection it has to be borne in mind that the Poles no less than the Russians are ruthless where human life is concerned-the Polish Government acted most unwisely in invoking the aid of the International Red Cross.
Our short term aim must be to close this breach in the United Nations solidarity as rapidly as possible in order to deprive German propaganda of the fruits of its victory and to reduce the risk of internal divisions of public opinion of the U.S.S.R.
especially in the United States. If Stalin is to be persuaded to resume his suspended relations with the Poles, both British and American action will be necessary and the President has made a good start by sending a most understanding telegram to Stalin.
Even if we can re-establish a facade, however, there remains a much greater fundamental problem. The U.S.S.R. is still, I believe, acutely suspicious of the Western powers and fears that she will be excluded from the post-war comity of nations. In this atmosphere we can only expect a progressive deterioration of relations between the Soviet and the British and Americans. The only way to obviate this is to make a bold approach to the political problems of the post-war world, particularly Europe, and for the British and Americans to work out with the Soviet as soon as possible the broad principles of the future political set-up.