We've already discussed 51% attacks on Bitcoin in a few recent threads. My own scenario had a large nation-state player like China or Russia doing it.

It's worth noting that in that discussion, it was pointed out that only two companies control more that 51% of the Bitcoin hashrate. An entity controlling these two companies (either through legal/political or clandestine means) could effectively control the Bitcoin network.

The other conclusion from the threads was that Bitcoin is ultimately protected by... the US federal government. Any actor, be it private or national, would have the USA to contend with if it were to try something like this. Indeed, it is undoubtedly the threat of the US government's reprisals that probably keeps a major entity from attacking Bitcoin. (Kind of ironic, given all of the anti-government rhetoric coming from some of the Bitcoin community).

This is worth thinking about, since the one thing Bitcoin could not survive would be an attack by the US government itself, say if Trump were elected and he was told that replacing Bitcoin with his own personal currency would make him a trillionaire.