Stalin arrived at the Tehran conference just after crushing the Germans at Kursk (the biggest Tank Battle in History). He bullied and dominated Roosevelt, by then a very sick man, while Churchill concentrated on British interests in the Middle East and Mediterranean (despite what his officials advised). Stalin got just about everything he wanted; a chunk of Poland, a puppet government in Yugoslavia and permission to dominate Eastern Europe. Roosevelt was a weak man, mentally as well as physically, but that doesn’t excuse the sell-out of millions of people. If the British and Americans hadn’t opened a second front the whole of Europe would have been under Stalin’s boot. There’s no chance Churchill didn’t know that before he arrived in Tehran (after Kursk).