In the case of Stalin, it is even more preposterous. He, given his underlying paranoia, would have undoubtedly have seen a major diplomatic push by the Western Allies as their meddling in Soviet affairs, and would have likely backfired, very badly. I mean, what would the form of said diplomatic pressures have taken, exactly? Promises of shipments of goods? [then there should be some scripts/a mechanic for that] Otherwise you are simply referring to the Allies sending a blizzard of telegrams, phone calls, and diplomats the Russian's way--but how effective would that really be, in the end? Talk is cheap, and in Stalin's case utterly worthless.
I mean, take a look at Franco as well while we're at it. There was really nothing Hitler could have said to have changed his mind--the things Franco wanted were the things Hitler/Moose simply were unable to deliver.
If major country diplo was limited to one chit it would be much more realistic. In 1940 neither the Soviets or US were going to enter the war with Allies. On the flip side with war raging all around him Stalin definitely wasn't going to reduce his military spending at all and certainly wouldn't have reduced it to zero which is possible here.
You see things; and you say “Why?” But I dream things that never were; and I say “Why not?”
George Bernard Shaw