If I would be a nation state opposed to bitcoin and use cyber capabilities against it, I wouldn't run DDoS attacks. I would hack MtGox and steal their coins. Both China and the US should have the capabilities to do so. If all the bitcoins of the exchanges would be constantly stolen.

The botnet's that run the DDoS on MtGox are probably payed via bitcoin by someone who spectulates on the price changes that the DDoS attacks cause.
DDoS attacks are cheap. You don't need to be a nation state to run a DDoS. You just need to pay a small sum of money to rent a botnet.

But back to a nation state attacking bitcoin, cyber attacks aren't needed. The US could simply start suing all bitcoin miners for facilitating money laundering and violating the principle of know-your-customer.

Announcing such law suits would alone do a lot more damage to bitcoins price than the DDoS attacks against exchanges. If current laws aren't enough congress can always pass additional laws.

The world doesn't look the way it would if the US government would make an attempt to shut down bitcoin. The US government isn't filled with the smartest people but if those people would go after bitcoin they could still do serious damage. Damages that exceed the damage that a single individual can cause by rented a botnet with his bitcoins.
Actually the US is perfectly able to attack other groups without any plausible denial ability.