Starting with the premise that the US Government tries to take down bitcoin entirely. 

What would be the most likely form(s) of attack? 
Would they be able to keep any viable form of attack secret or shift blame to another entity? 
Which if any methods do you believe would have a likelihood of success given a concerted effort? 
What would very rough dollar cost estimates be for various methods?
Would there be signals of an attack or could this occur with no warning?
Could any responses be developed within the bitcoin framework or do you see another existing or proposed cryptocurrency that would be safe from such attacks?

Sorry I know it is a messy bunch of questions, but perhaps also the most important to consider long-term.