I think it would be more than the US, it would be nearly all governments with a central bank.
Play that scenario out to its conclusion. You're suggesting multiple countries would band together, to form a federated mining pool, to 51% attack the Bitcoin network ? Spend God knows how much in development, deployment, and energy costs for ASICs ?
I don't see it happening.
I guarantee china and israel have been trying very hard to find any exploits with BTC for a while.
Every safe can be cracked, brute force is one method but not the only one.
I can guarantee you some basement dwellers have been trying very hard to find any exploits with BTC for a while now.
We don't know how resistant to exploiting BTC is, but so far, it's lasted this long without any serious problems (unlike Ethereum smart-contracts getting exploited and funds extracted)
Trying to brute force a wallet key via hash-collision would take many millenniums with available supercomputing, and there has been some debate that there is a certain degree of quantum attack resistance, but it's all speculative at the moment.
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