It was definitely Soviets/Stalin that started conquering and this was a western reaction to it. The Yalta conference was an attempt to appease Stalin and give Russia the conquests it wanted (it's not like they appeased someone else 10 years before leading to the incredible disaster, arguably the biggest disaster in human history, or anything like that. But not giving up Eastern Europe would have likely meant war with the Soviets right then and there)

It was a choice between giving up a lot, not just Poland, and continued war. Must have sucked pretty damn badly if you were one of the things given up to Stalin, but ...