This came up as a side discussion in another thread, and I thought it would be interesting as a top-level discussion here.

The notion is this: although taking over 50% of the Bitcoin hashrate would be a practical impossibility for any private actor or even a small country, China, as a country, has at times hosted over 50% of Bitcoin's hashrate, and it's thus plausible they could make this happen again if they wanted to.

And whomever controls over 50% of the hashrate can dictate the software in use, and/or dictate the contents of the blockchain. As the theory goes, if China wanted to wreak havoc on the world economy, or if it wanted to extract a ransom, or if it simply wanted to profit by inserting its own blocks, then... it could. They could do anything. Perhaps China could decide that Satoshi's blocks should be given away to charity. Maybe they change the architecture so that it worked better in China, or conformed to their "laws" or something.

So this brings up questions:

1. Is this technically possible?

2. What would be the technical implications of China doing this?

3. What would be the geopolitical implications?

4. What would this do the broader cryptocurrency sector?

5. Have major governments e.g. the US and NATO countries ever (publicly) announced any contingency planning around this problem? Putting a $1.5T asset at risk is something that could rattle the entire world economy. This seems like something they would at least think through?

Looking forward to people's thoughts...