Refer to Bitcoin Jesus's video below. He says it quite well.
https://www.youtube.com/watch?v=ncPyMUfNyVMThere is no way that a government gets hold of and installs millions of antminers, powers them and manages to sustain the attack. Think of the logistics behind it. Consider how much electricity they will need, they will probably need to power down their entire country, and after all that they can just do one double spend.
thats a great post, however there is also non governmental threats, and so called de facto centralisation, if big miner pools associate themselves.
Well let's say Antpool wants to be malicous. How long do you think it'l take for that to get noticed and word to spread? The pool is centralized, but remember it is made up by thousands of individual miners. They will instantly leave the pool and mine elsewhere.
This has happened in the past when one of the major pools were approaching 50% hashrate. Word spread fast and people left the pool. All individual miners will act to protect the network even within one pool. If a pool attempts an attack they will probably fail to get sufficient hashrate, and will lose all their miners before the attack can even really happen.