They didn't have one in time to use. V-E day was May 8, 1945. The first atomic bomb was tested July 16, 1945. They were going as fast as they could at that point.

If they had had one in late 1944, during the Battle of the Bulge, might they have used one? Maybe. It wouldn't have been the easiest logistical operation, and the situation with Germany at the end of the war was very different than with Japan. But in any case, this is a moot point, since they didn't have one.

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