On March 17 [1939] the British Government suddenly remembered the existence of the Soviet Union and inquired what its attitude would be toward the Hitler threat in Eastern Europe. Moscow replied promptly and proposed an immediate conference between Britain, France, the U.S.S.R., Poland, Rumania and Turkey to consider how to resist German aggression. This was exactly and obviously what was urgently needed. Nothing less than a drawing together of all the threatened states could be of any avail. Rumania was under intense pressure to turn over her economy to Germany and the quickest action was needed.
  However, the Chamberlain clique could not make the shift. Knowing that they wanted to solve the Nazi menace on the plains of Russia, they ascribed to Russia the very same design of which they were guilty, as devious men so often do. They had a “deep seated conviction,” shared also by the French Rightists, that Russia wished to destroy the capitalist system in Europe by provoking a war from which she would remain aloof.5 The entire diplomatic record of the past five years belied this self-justifying suspicion. It showed that Russia was desperately anxious to avoid war, but also that on every occasion, without exception, she had sought to avoid war for herself by combining with others to prevent aggression or nip it in the bud. It was Russia which had incessantly pleaded that “peace is indivisible,” warning that if war came all would be engulfed in it.
  All this had meant nothing to conservative men bent on making terms with fascism and preserving it. Now, therefore, Chamberlain hesitated a week until Rumania capitulated and on March 18 notified Russia that her conference proposal was “premature.” This was the same rebuff given to Russia when she had proposed a conference a year earlier, at the time of Hitler’s conquest of Austria. Again there was no hurry, but this time Chamberlain did propose a substitute plan whereby Britain, France, Russia and Poland would consult if any further acts of aggression were believed to be imminent, but even this proposal was abortive, since it was at once learned, says Chamberlain’s biographer, that “Poland would refuse contact with the Soviet, which alone was enough to prevent us from taking up the Russian proposal for a six-power conference.” Chamberlain did not blame the Polish Government. He confided to his diary, on March 26: “I must confess to the most profound distrust of Russia,”6

Poland’s comfortable blocking of any liaison with Russia revealed the impossibility of stopping Hitler, at this late date, without a gigantic world war. Poland was now the prisoner of her conquests. Obsessed by delusions of grandeur, she had conquered from Russia some ten millions of White Russians and Ukrainians whom her great landlords did not dare to trust in contact with Russian troops, even if they were fighting for Poland’s very life. Poland had also aggrandized herself at the expense of Germany, Lithuania and Czechoslovakia, acting like an eager and vindictive vulture when Czechoslovakia was being dismembered. The cause and result of this attitude toward her neighbors was that the splendid Polish people were “ruled by an incompetent and purblind oligarchy who preferred government by junta rather than by parliament.”7 This oligarchy of landlords and colonels also believed itself to be a Great Power. Impressed by its conquests, it adopted the impossible policy of balancing both of its huge neighbors against each other. Colonel Beck’s slogan, “Not a millimeter nearer to Berlin than to Moscow,” would have been valid under a strong League of Nations, but it meant certain destruction in the midst of an anarchy of aggression on the loose. In this situation Poland might survive by close alliance with one of her great neighbors. Being unable to choose between her hatreds doomed her to sure destruction and in all probability to another partition.

On that date [April 15] the British Government asked Russia to give a unilateral guarantee to Poland and Rumania. What good this would do, since Poland would not accept Russian assistance, was a mystery, but if that obstacle could be overcome London was willing for Russia to do the fighting. In reply to this transparent maneuver, Russia promptly proposed, on April 17, a binding pact of mutual assistance between Britain, France and Russia, to be implemented by a military agreement, which would guarantee all of the border states from the Baltic Sea to the Black Sea

  The Soviet Chief-of-Staff, Marshal Voroshilov, outlined a plan to block German pincer movements through Northern Poland and Lemburg in the South. The Allies conferred with Poland and, after four months of negotiations, with German troops piling up on her borders, Poland replied that she did not need Soviet aid.
  According to Gafencu, Paris put the strongest pressure on Colonel Beck, but he returned a firm “No.” He believed the U.S.S.R. to be too weak militarily to be of much help, but if she were strong she would never evacuate the region occupied. He would not “admit that, in any form whatever, the use of our territory by foreign troops should be discussed.”

(Bold added by me)

The Cold War And Its Origins, 1917– 1960. Vol. I, 1917– 1950 By Denna Frank Fleming, Chapter 5