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Suppose The Sum of All Fears scenario occurred during World War II, with a nuclear armed B-29 crashing (they were quite unreliable) or being shot down over Imperial Japan. The PoD could be anything from there being a third bomb lost to history, one of the two historical bombs being lost, or a weapon being lost during the Operation Downfall invasion.
Could Imperial Japan turn the captured device into a usable weapon, especially if it is a uranium type weapon? Where would they deploy such a weapon? Going into a plot more akin to The Sum of All Fears, what if the weapon was discovered after the war? What would a group of holdouts or radicals use it for? Would there be a way to use it to spark nuclear war between the Soviet Union and United States?