How difficult (expensive, realistic, etc) would it be for the US gubmint to buy up enough mining power to hijack the network, change the rules, and send it all to zero?
In bitcoin history, no one has successfully done a 51% attack successfully being it an individual miner or group. Only if one has more than half percentage of the hash-rate the attack can probably be possible. It's not definite that you'll reverse the next block, but it's a matter of time until you attack successfully.
When a miner possesses 51% of the hashrate and having it mine in secret a second chain is close to impossible right now, because 20% of the hashrate is controlled by public companies (which is 100% likely to increase in the near future). This won’t be the only first bankruptcy but will also face lawsuits after lawsuit for this act.