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.