Just make it a NO for the US that it needs a cumulative die roll to obtain nukes
Certain things have to happen in order for the US to roll “Atomic Dice”
Allied control of Rome
No Axis controlled territories in Africa
The Axis cannot control US originally controlled territories in the Pacific (Guam, Midway, Hawaii, Johnston, Line, Philippines, Aleutian)
The Axis cannot control UK originally controlled territories in Europe (London, Scotland, Iceland, Gibraltar)
Anzac has to have collected IPC that round
Before Germany goes in that round, the US gets to roll a die for each of the following:
US Control of Rome
US Control of Hawaii
US Control of Caroline Islands
US Control of Marianas
US Control of Iwo Jima
US Control of Okinawa
US Control of Marshall Islands
France collected IPC that round
Allied Control of Egypt
You have to roll something like six sixes in order to obtain an atomic bomb. Thats six sixes cumulatively. Alternately, you can make it a cumulative total so something like you have to roll something like a die total of 40-50 cumulatively on the die roll(s) over multiple rounds.
Each nuke thereafter costs the US 1/3 of its IPC to purchase it (so probably something like 25-30 IPC at this point in the game).
The Nuke works just like a SBR however it can only be on the Nation’s Capital (Tokyo / Berlin). If the StratB’s with the Nuke(s) get to the roll dice phase for damage to the Major IC, then the attack is successful.
A successful attack means that nation does not collect IPC that round.
From the German perspective:
Germany rolls a single die every round for the following objectives:
German Control for each VC it controls
So G1 would see Germany likely roll die for Berlin, Warsaw and Paris.
Germany would also be able to roll die if it controls London, Egypt, Leningrad, Stalingrad, and/or Moscow which allows for Germany to go in any direction to obtain extra die rolls.
Germany scores half (rounded down) its total die roll if the Axis does not control Paris and Rome.
Germany has to obtain the same objective as the US to obtain an Atomic Bomb (six sixes or something like 50 total showing on the dice cumulatively through all rolls).
I believe this puts the Atomic Bomb in play for Germany around turn 7-8 if it never loses Paris and owns London. However Germany could only really play that Atomic bomb against Moscow if it sealioned G3 as its pretty darned difficult for it to launch a SBR against the US. In effect it penalizes Moscow for sitting back and really doing nothing instead of pushing back against the Germans.