Why did Britain and France declare war only against Germany when the Soviets also invaded Poland?

because everyone knew the Soviets and the Nazi's were going to start fighting sooner or later,

if they declared war on the soviets AND the nazi's that would simply have forced them both to work together for longer,

instead they left it, and within 18 months the Soviets and the Nazi's were killing each other

Ha, you strike a superb component! maximum individuals look blind to the undeniable fact that the Soviet Union signed a peace treaty with Nazi Germany, and that they the two invaded Poland on a similar time - Germany from the West, the Soviets from the East. They divided up the territorial proceeds. The Soviets weren't the "good adult males" till that they had an user-friendly enemy with Britain. we are able to in basic terms anticipate the Western alliance on the time (France, uk et all) in basic terms had a quarrel with Germany - past to that, the Germans were continuously annexing land and posing a probability to them - this became lesser so with the Soviets. and probably they only did no longer opt to chew off greater beneficial than they could chew. France and Britain have been lifeless afraid of a conflict with Germany on the time, no longer to point a conflict with Germany and the Soviet Union.