It just strikes me as odd that the war against Germany is seen as a necessary (and righteous) response to its expansionism, whereas the USSR was ignored and then treated as an ally of convenience. Did the Soviet Union as the lesser of two evils, or was a war against both considered infeasible, or what?
The Anglo-Polish Agreement of 1939 seems pretty clever. Both sides agreed to come to each others aid in the event either was attacked by a European power. A secret addendum specified that the European power in question was Germany. It was a formal reiteration of the verbal assurances given to the government of Poland in the wake of the Munich Agreement.
On the face of it Britain could make the age old distinction of Russia as being an Asian power, rather than European. At the time of the partition of Poland, allowing the Soviets to take the other half of the country was seen as a good counterweight to Germany's expansion in Eastern and Southeastern Europe. The abortive plans to bomb Russia's oil production at Baku were shelved as they would only exacerbate the situation further.
It's the concept of "The enemy of my enemy is my ally". By 1939 Germany had rapidly become a military powerhouse and there were real concerns that the Allies at that time would lose the war. The British and French were willing to overlook ideological concerns to defeat Germany. An alliance with the USSR did a number of things. It opened up a second front and gave the Allies a massive amount of allied soldiers. Also there was no way that Britain and France would have been able to take both Germany and the USSR.