This point needs to be stressed. It took less than 5 years for the Soviets to figure out the nuclear bomb, despite the great novelty of the idea, the many unknowns surrounding nuclear physics at the time, the scarcity of the ingredients, and the fact that U.S. research was conducted under total secrecy.
Meanwhile, the entire international community of scientists has been thinking about fusion for decades, publicly sharing their results, and scientists in every country know what the rough obstacles are. A big leap forward in fusion wouldn't even take 5 years to disseminate across the community, it would take weeks.
Sure, and nowadays any big science project has hundreds if not millions of "spies". You, too, can dig into the data and methods of many such experiments, no matter what country you liv in.
Meanwhile, the entire international community of scientists has been thinking about fusion for decades, publicly sharing their results, and scientists in every country know what the rough obstacles are. A big leap forward in fusion wouldn't even take 5 years to disseminate across the community, it would take weeks.