We only had 2 nukes, and they were both used on Japan. In fact, Japan was still fighting after Nagasaki, and only surrendered after we lied that we had a 3rd bomb ready to level Tokyo.
So we would have had to invade Germany, which would have costed twice as many lives if we had to invade Japan.
There were supposedly a total of six bombs [four new Plutonium bombs plus the Uranium and Plutonium bombs already dropped on Japan] when Oppenheimer left Los Alamos at the end of September, 1945. This fact was roughly verified by the Russians, who were apparently told by Klaus Fuchs that the Allies could "threaten them with no more than five bombs."
Four or five bombs could have destroyed Germany or the Soviet Union for all practical purposes in 1945.
Those nukes were meant to be dropped on Germany, but Germany was defeated before they were ready. We didn’t start a nuclear weapons project because we feared Japan had one. We developed them because we thought we were in a race against German nuclear scientists.