China

The capitalist bloc has too much investment in fossil fuels (with the petrodollar being the very foundation of the capitalist bloc’s world dominance) to let an “unlimited energy source” propagate around the world. Especially not if this source was controlled by a socialist country.

As I think about this such a development could be a big reason for the US to use nuclear weapons against China, so I do hope China builds something like laser defenses to take advantage of the new power source.

In my opinion, only the creation of a world socialist federation (not literally every country in the world, but a significant number) could create the political conditions for fusion power to become humanity’s main energy source. It would be necessary not only to get rid of political opposition from the capitalist bloc, but also to provide generous terms on which the third world could acquire fusion energy without having to undergo carbon intensive development first.

This could go hand in hand with cheap fusion reactors though, because if the reactors become very cheap fast and China can mass produce not dozens per decade but thousands per year this completely changes the game in a way that no one would really care for oil (other than the US and a few countries reliant on oil sales) in a year.