Could a bunch of the scientists be killed in a major accident, say in 1942 or 43 that sets the project back by several years, to where even a gun-type will be delayed until late 1946 at the very earliest.
Combine that with a Stalin who makes a Brest-Litovsk II treaty (not as punitive as CalBear's but on par or a little nastier than the first one), and you could see quite a Fortress Europa that's hard to crack.
Combine that with a Stalin who makes a Brest-Litovsk II treaty (not as punitive as CalBear's but on par or a little nastier than the first one), and you could see quite a Fortress Europa that's hard to crack.