A malicious government could totally make a 100mW data center, fill it with hardware and "attack" the network. I don't see how they could not. Now if a government is going to want to sign off on such a expensive, open attack.. i very much doubt.
Doing this would piss of a lot of people.
There's probably more covert ways to go about this.
Is it generally certain that all current hashrate is more or less accounted for, and not, in fact, a malicious government running some operations covertly right now?
How would a government building a big data center be some malicious operation, if its pointing its hashrate at a legit mining pool? You're wondering if they just plan on mining a ton of BTC and then dumping it to hurt the value?
Or they're just mining to cut their losses while they spool up some their 1 exabyte datacenter? Well they're better hurry up, because soon, that won't be enough.
It would be much profitable for them to just earn through mining instead of dumping to hurt bitcoin. They can't win this battle anyway so they might just wanna join with us. This a is much better option for them to which they should think about.
They definitively, easily can "win this battle". Its not hard to crash the price of Bitcoin if you have billions of dollars alloted to doing this. The feasibility is not the problem. Its the inclination and the international impact.
There is no point to do this if you're some lone country that goes against all other countries and attack Bitcoin. And if all the countries want to get rid of Bitcoin, then they can just consensus to outlaw it at that point. Something that does not do much when you're a lone country that decide to outlaw it. It would make more sense than spend a shit ton of money.
This is scare speculation. There is nothing to support this other then saying it could happen. I mean most goverments would have no interest in doing this, as what's in it for them? Unless they have a crap fiat not much.
Largest we know of is US goverment with Silk Road. And they did it as a auction on BTC, was fair and did not hurt BTC that I saw. So if goverments do like one of the few examples we have it really is not that scary. They want USD or fiat not BTC.