If someone has access to primary sources on discussions related to this topic, that would be helpful. However, I can provide context that might help give understanding.

The answer is rooted in the events leading to the invasions of Poland. The diplomatic dynamics among the Western democracies, Fascist states, and the USSR are very important.

Most familiar with the diplomatic achievements of Hitler between 1933 and 1939 focus on the response of the Western democracies. When Germany expanded its army beyond 100,000 and re-militarized the Rhineland, France had desired some sort of concerted military response; however, British thought had begun to consider the Treaty of Versailles differently and had begun advocating Appeasement (based on the idea that allowing Germany to regain "losses" to the Treaty would both avoid war and right the wrongs of the Treaty). Hence, the British did not support French plans and the French did not want to act alone.

During the Sudetenland crisis, in which Hitler demanded that Czechoslovakia cede the Sudetenland to Germany, appeasement again won the day and Chamberlain, British prime minister, essentially forced its ally, Czechoslovakia, to cede its land or fight a war against Germany without support by Britain and France. A few months later, German forces entered Czechoslovakia and took the entire state.

What is less well known is that Czechoslovakia also had an alliance with the USSR. Throughout the Sudetenland crisis, Stalin had been supportive of a concerted effort by the USSR and the Western democracies to protect Czechoslovakia. He even supported war against Germany in the case of an invasion. Yet, he was also fearful that the Western democracies wanted to use the USSR against Germany in a way that would weaken them both and strengthen the position of Britain and France. Hence, the USSR would only support Czechoslovakia if Britain and France did as well.

When Stalin was left out of the Munich Conference, this only reinforced the belief that the Western Democracies were working with the Fascists against the USSR. He then made the Molotov-Ribbentrop Pact with Germany. It divided Eastern Europe into two spheres of influence, one German and one Soviet.

Finally, when Germany invaded Poland, Britain and France declared war. Only weeks later did the Soviet Union invade Poland. Britain and France could have then ALSO declared war on the USSR, but this would have only been detrimental to the war effort. The failures of appeasement had demonstrated that Hitler was not to be trusted and made Germany to be seen as the primary aggressor.