“ It is clear that Stalin had two courses open to him. He could seek a general coalition against Hitler, or he could come to an understanding with Hitler at the expense of the Western democracies. Stalin’s policy was guided by a profound conviction of the ultimate hostility of Nazi Germany, as well as by the hope that if the capitalist Powers became locked in mortal conflict, the Soviet Union might remain aloof, gaining strength while they tore one another to pieces. Certainly the principle of self-preservation lay at the heart of Moscow’s calculations ” Winston Churchill, The Gathering Storm, Chapter 20, The Soviet Enigma pub 1948.

“ In those days the Soviet Government had grave reason to fear that they would be left one-on-one to face the Nazi fury. Stalin took measures which no free democracy could regard otherwise than with distaste. Yet I never doubted myself that his cardinal aim had been to hold the German armies off from Russia for as long as might be ” (Paraphrased from Churchill’s December 1944 remarks in the House of Commons.)

“ It would be unwise to assume Stalin approves of Hitler’s aggression. Probably the Soviet Government has merely sought a delaying tactic, not wanting to be the next victim. They will have a rude awakening, but they think, at least for now, they can keep the wolf from the door ” Franklin D. Roosevelt (President of the United States, 1933–1945), from Harold L. Ickes’s diary entries, early September 1939. Ickes’s diaries are published as The Secret Diary of Harold Ickes.

“ One must suppose that the Soviet Government, seeing no immediate prospect of real support from outside, decided to make its own arrangements for self‑defence, however unpalatable such an agreement might appear. We in this House cannot be astonished that a government acting solely on grounds of power politics should take that course ” Neville Chamberlain, House of Commons Statement, August 24, 1939 (one day after pact's signing)

“ We could not doubt that the Soviet Government, disillusioned by the hesitant negotiations with Britain and France, feared a lone struggle against Hitler’s mighty war machine. It seemed they had concluded, in the interests of survival, that an accord with Germany would at least postpone their day of reckoning ” Cordell Hull (U.S. Secretary of State), The Memoirs of Cordell Hull (Published 1948)

“ *It must be said that the Soviet Government, having little confidence in swift military aid from the Western Powers, chose to protect its borders, however odious such a pact might seem. One perceives in their choice the determination to secure time—time they evidently believed we were not prepared to give them.” Édouard Daladier (French Prime Minister), Address to the French Chamber of Deputies, Late August 1939

“ It seemed to me that the Soviet leaders believed conflict with Nazi Germany was inescapable. But, lacking clear assurances of military partnership from England and France, they resolved that a ‘breathing spell’ was urgently needed. In that sense, the pact with Germany was a temporary expedient to keep the wolf from the door ” Joseph E. Davies (U.S. Ambassador to the USSR, 1937–1938), Mission to Moscow (1941)

“ British officials, for all their outrage, concede that Stalin, with no firm pledge of Allied assistance, and regarding Poland as a foregone victim, decided that if the Red Army must eventually face Hitler, it should not be without first gaining some strategic space—and time ” Joseph P. Kennedy (U.S. Ambassador to the UK, 1938–1940),Private Correspondence, September 1939

Hopefully, you won't accuse such sources, i.e. western diplomats and politicians who actually experienced WW2, of being tankies