No one except for the owners of major BTC mining operations.
If a state actor, such as China, was motivated enough to identify, capture and coerce these miners it would be pretty easy to perform a 51% attack given that China accounts for around 65% of all bitcoin mining globally.
Yes, and the elephant in the room with Bitcoin is that it's only going to get easier. Despite the categorization of Bitcoin as non-inflationary, the computational power of the network is currently ~85% subsidized by block rewards that grant miners new coins (i.e., inflation). Once those are diminished through halvings, either transaction costs will have to go up about 7x to match the current incentive, or the network difficulty will come down as miners go offline. If the latter happens, not only will the hash rate go down, there will be a flood of mining hardware on the market as miners exit the game.
Oh, and if “layer two” solutions ever do take off, there will be downward pressure on transaction fees, too.
That Bitcoin is some immutable “backed by math” financial system has been a charade all along. It’s very much backed by people, and proof-of-work is a Rube Goldberg machine used to distract the tech-literate.
Ultimately, Bitcoin is backed by node operators and widespread social consensus. Thinking China can just split and control the network is a pretty serious misunderstanding of the power dynamic in this system.. one we all had demonstrated at the conclusion of the block size wars.
Satoshi wanted it to be backed by Proof of Work, but now you’re saying Bitcoin is backed by social consensus... just like the US Dollar I guess?
Or they just deploy their own mining pool.
Or the just tax all of their citizens’ cryptos.
They have lots of options, since they have the power of the state behind them.
Soft and hard forks are a defensive mechanism against 51% attacks, essentially making them an always losing proposition outside of the immediate-term where the attack is occuring, because the attacker has to expend so much money, and then a simple code change could revert it, which would likely happen in consensus for the network if such a malicious attack would occur. Hence, no rational actor will likely even consider a 51% attack.
This is because people don't consider it likely. Once it happens, people will move to new crypto and will devalue any crypto that is too centrally mined by a single country.
Currency flight from China is a serious problem, and the CCP has instituted controls to make sure money doesn’t leave the country (except for some permitted amount)
I don’t think it’s unlikely that BTC gets cracked down on by the CCP if they notice that a large volume of citizens are using it to skirt capital controls.
What matters is public perception of if it could happen, not if it can happen.