The Hitler-Stalin Pact as seen by the Master of Realpolitik:
Driven by moral outrage and strategic confusion, Great Britain thus slid into guarantees on behalf of countries which all of its postwar prime ministers had insisted it could not, and would not, defend. The post-Versailles realities of East-ern Europe had grown so remote to the British experience that the Cabinet did not even realize it had made a choice which would multiply Stalin's options toward Germany and ease his withdrawal from the proposed common front.
In fact the British guarantee to Poland and Romania removed whatever incentive the Soviets might have had to enter into a serious negotiation about an alliance with the Western democracies. For one thing, it guaranteed all the borders of the Soviet Union's European neighbors except for the Baltic States, and, at least on paper, thwarted Soviet ambitions as much as it did Germany's.
But, more important, the unilateral British guarantees were a gift to Stalin because they provided him with the maximum he would have asked for in any negotiation which started, as most negotiations do, with an empty slate. If Hitler moved east, Stalin was now assured of Great Britain's commitment to go to war well before the Soviet frontier was reached. Stalin thus garnered the benefit of a de facto alliance with Great Britain without any need to reciprocate.
Poland could not be induced to accept Soviet help, because its leaders were convinced (correctly, as it turned out) that any "liberating" Soviet army would turn into an army of occupation.
The Soviets' interest in preserving the status quo in Eastern Europe ended with the Eighteenth Party Congress—if, indeed, it had ever really existed. Crucially, Stalin did in fact have the option of turning to Hitler and, after the British guarantee to Poland, could play his Nazi card with considerable safety.
His task was eased because the Western democracies refused to grasp his strategy—which would have been quite clear to Richelieu, Metternich, Palmerston, or Bismarck. Quite simply, it was to make certain that the Soviet Union was always the last major power to commit itself, thereby achieving the freedom of action for a bazaar in which either Soviet cooperation or Soviet neutrality would be offered to the highest bidder.
Before the British guarantee to Poland, Stalin had had to be wary lest Soviet overtures to Germany cause the democracies to wash their hands of Eastern Europe, leaving him to face Hitler alone. After the guarantee, he had an assurance not only that Great Britain would fight for his Western frontier but that the war would start 600 miles to the west, on the German-Polish frontier. Stalin had only two remaining concerns.
First, he had to make certain that the British guarantee to Poland was solid; second, he would have to find out whether the German option really existed. Paradoxically, the more Great Britain demonstrated its good faith with respect to Poland, which it was required to do in order to deter Hitler, the more maneuvering room Stalin gained with respect to Germany. Great Britain sought to preserve the Eastern European status quo. Stalin aimed for the greatest range of choices and to overturn the Versailles settlement. Chamberlain wanted to prevent war. Stalin, who felt war was inevitable, wanted the benefits of war without participating in it.
Molotov demanded that all the countries along the Soviet Union's western border be guaranteed by both sides and that they be specifically enumerated (ensuring a formal refusal from at least some of them).
He also insisted that the term "aggression" be expanded to cover "indirect aggression," defined as any concession to German threats, even if force had not actually been used.
Since the Soviet Union reserved for itself the definition of what was meant by "yielding," Stalin was also in effect demanding an unlimited right of intervention in the domestic affairs of all the Soviet Union's European neighbors.
By July, Stalin had learned enough. He knew that the British leaders would consent—however reluctantly—to an alliance on close to his terms. On July 23, the Soviet and Western negotiators agreed on a draft treaty that was apparently satisfactory to both sides. Stalin had now acquired a safety net for determining exactly what Hitler had to offer.
Foreign Policy Crisis -Stalin's Bazaar by Henry Kissinger