The US went first. Stalin heard about it after Churchill but before Truman.
Hitler and Tojo had heard about the idea but thought that the war would not last long enough. Tojo was wrong. The US established two very costly production lines.
Later Britain traded the secret of VX for the Teller-Ulam solution and a couple of submarines. The US never used the VX and Britain never employed the Teller-Ulam solution.
De Gaulle was not one to take a back seat. Never took much advantage.
Mao put Deng in charge of the Chinese response. China clearly believes that Japan is an undeclared Nuclear Power and relations are touchy at best.
India was not willing to be left behind after China.
Pakistan took notice. It appears that Pakistan sold some of the secrets to Syria, North Korea, and Libya. It is possible that one of Pakistan's sponsor's was the House of Saud.
Brazil, Argentina, and South Africa probably worked on the problem.
The best good news is that no non state actor has gotten a Gadget of any size.
Is there any nation willing to make the second first use?