Among the many feasible vectors with which you could accomplish it: Bitmain's HQ is in Beijing, and while there is some scrutiny on the subject there has never been a (public) investigation into what's actually baked into the silicon to check and make sure there's not a 1337 backdoor or similar. Antminers are a fucking stunning percentage of the nodes out there, and although nothing like 51% of all nodes are just antminers, it still wouldn't be difficult (for a powerful nation-state) to repeat the process across multiple manufacturers and get that kind of distributed influence.

It's completely possible that the classically imagined 51% attack has never occurred (the "muhahaha all your coins are belong to us" scenario), but having that kind of large influence over the blockchain would trivially allow for subtle manipulations that would have world-spanning impacts. We have no way of demonstrating that it's happened, nor mitigating it if it has, which is a big part of why bitcoin is regarded as such a joke by everyone that isn't part of the cult. And to be clear, I have no idea if this has happened - that's the problem, there's by design no way of checking or verifying integrity on that scale.

Bitcoin was a half-baked idea that's only stuck around because of the speculative market.