Harry Truman after dropping the bombs on Japan did not want to use it again. Chinese spies in his administration knew in advance he was not going to use it if they crossed the Yellow River. If Truman instead had been looking forward for any excuse to nuke Communism to hell the Korean war would have ended early. The point is we already know what the reservation not to use nukes actually leads to because it already happened.
Just imagine what the world would be like if Truman had established a policy in 1945 --
"Nuclear weapons are a terrible thing, and no one should have them. Therefore, the USA will have them. And the USA alone. Any nation that tests a nuclear weapon -- even our great ally the UK -- will have its capitol city nuked within 24 hrs. This is how we intend to stop nuclear proliferation. So, let the message go out -- to the USSR, to China, to all nations: no one should seek to develop nuclear weapons, or they will face the consequences."