Why an American nuclear monopoly would be hard to preserve

What if the United States remained the only country with atomic weapons?

It would gain exceptional power, but allied nuclear knowledge and the political cost of suppressing it would make the monopoly difficult to sustain.

A lasting American monopoly on atomic weapons would create overwhelming strategic dominance and might make further use more likely. Britain was also well placed to develop a weapon because it had conducted substantial prewar work and combined that effort with the American programme during the war. The later American decision to restrict cooperation did not erase the knowledge already shared. Preserving the monopoly by attacking Britain or France immediately after fighting alongside them would be politically untenable at home and abroad. The hypothetical monopoly is therefore easier to imagine than to enforce against allied nuclear programmes.