As I understand it, if the US Govt chose to entirely eradicate bitcoin worldwide the most likely way would be through a 51% attack (please correct me if some other software-based attack might be more effective).
It wouldn't completely eradicate Bitcoin though. It would just shake people's confidence in it and require some very, very ugly solutions.
Mining pools could sign their blocks and could agree not to build onto a block that wasn't signed. Clients could prefer signed blocks to unsigned blocks in reorganizations. One or more distributed, high-speed checkpointing systems run by trusted individuals could be set up. Path dependency could be added to the "longest chain wins" algorithm. The mining algorithm could be changed, forcing the attacker to start over from scratch.
Thanks, that is encouraging. However, during the time period while an attacker had control of 51% of the cloud, could they destroy/redirect all records of existing bitcoin ownership - or new ownership going back ___ (please estimate) weeks? Could an attacker hide within mining pools or is it certain the offending nodes could be detected and removed?