Russia: Petlyakov Pe-8
Germany: Heinkel He 177
Britain: Avro Manchester
France: Farman F.220
Japan: Mitsubishi Ki-20
USA: Boeing B-17
Italy: NONE
These are your only airborne options for weapon delivery at the onset of war in 1939. No other aircraft at the time had the carrying capacity for a 4,400 kg weapon.
People saying the Japanese should bomb San Francisco, or that Germany should bomb Magnitogorsk, are forgetting that the Axis NEVER had the ability to bomb these cities from afar, and certainly not with a 4,400 kg weapon like a Little Boy.
The British and German bombers were at the prototype stage at this point, and might not be serviceable. The later British bombers (Lancaster, Halifax, Stirling) were all still at least a year away from service. The American bomber would be limited to a range of less than 400 miles, and the B-29 was still 3 full years away from it’s first flight.
In short, aerial delivery in this scenario is incredibly risky. Your plane is slow, overloaded, has short range, and is incredibly vulnerable to attack. The only reason America was able to bomb Japan historically in such a manner was it’s new high-altitude bomber and it’s total air superiority.
The most reliable delivery method in this hypothetical scenario is probably via submarine. While heavier than a torpedo, a nuclear weapon of the era was shorter and not much wider than a torpedo, so it is reasonable to assume a bomb could be placed in a sub with a volunteer skeleton crew and detonated near any number of targets.
Land based delivery is possible in some circumstances, but it’s probably pretty risky, especially if enemy units are around who might intercept your weapon.
We can pretend like advances might be made in missile technology, or more rapid bomber development, but I prefer to work within historical parameters, rather than make guesses as to what might be available.
In short, even with six bombs, it’s not going to be easy to utilize them effectively. I worked out some options for Germany, and might do other nations later. Germany’s nuclear focus is on Britains navy and Russian logistics.
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GUIDELINE - Do NOT confirm OR deny the possession of any additional weapons at any time. Let the enemy guess as to whether you have more weapons, and take his own risks accordingly.
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1. Scapa Flow - Before Oct 1939 - via suicide submarine.
This strike will take out a significant portion of the Royal Navy in it’s home harbor. The sub surfaces just prior to detonation to maximize damage. Historical precedent for this action exists. This has multiple positive outcomes for Germany. The British may very well call it quits after sustaining such a blow that early in the war. If not, they must call in ships from the Pacific and Indian oceans, along with the Mediterranean, to make up the loss to the Home Fleet for defense of the home islands. Convoys will be harder to guard with fewer ships available, and what’s left of the fleet might be relocated to a harbor either less secure from air attack, or farther from operational areas.
2. Dunkirk - On or about May 29 1940 - via suicide submarine
Surfacing as close to shore as possible before detonation, this attack strikes a fatal blow to the British Expeditionary Force as it attempts to evacuate the continent. This strike has the added benefit of dealing additional damage to Royal Navy assets, along with civilian watercraft aiding in the evacuation. The civilian casualties will likely mean a heavy blow to British morale. With the evacuation efforts cut off by the attack, nearly 200,000 British and French soldiers who would have otherwise been evacuated will be killed, sickened, or captured.
3. Suez Canal, southern entrance (city of Suez) - On Sept 9 1940 - via suicide submarine
Cutting the British Empire in two just as Italy begins it’s invasion of Egypt, this action will greatly hinder the already beleaguered British in their attempts to sustain their empire, lengthening maritime resupply routes and slowing the flow of Indian and Australian troops to the African theater. German subs can also now count on new hunting grounds along the African coasts. While not as decisive a blow as Scapa Flow, or Dunkirk, the aid to the Italian efforts should be significant enough to positively affect the outcome of their Egyptian campaign.
4, 5, 6. Murmansk, Arkhangelsk, Vladivostok - after June 22 1941 - via suicide submarine
Destroying these port facilities at the start of Operation Barbarossa cuts off the Arctic and Pacific options for Allied supply of the Soviet Union, limiting them to the Persian Corridor. Murmansk and Arkhangelsk might be within range of bombers stationed in Norway if the naval delivery option is ruled unfeasible. As at Suez, the residual radiation from the weapons will likely hinder reconstruction efforts for a significant length of time. Cutting off the Soviet Union from Allied aid in the initial phase of invasion gives Germany the best chance for success, far better than any single tactical detonation in a ground campaign or against an industrial center.