Once WW2 was drawing to a close, with Germany defeated and Japan not far behind, tensions between the leaders of the Allies were heating up. One 'buzz point' was Poland. The USSR had a very different idea of what should happen to it than the western allies, so much so that Winston Churchill actually drew up plans to attack Russia in order to enforce the West's will; this plan was called Operation Unthinkable.
It was well named, as it would have prolonged the war by years, caused many, many more deaths and likely would have included more atomic bombs. In the end, it was scrapped for being 'too hazardous'.