Like I said, if the US really wanted to stop Wikileaks, I believe it really could. I'm just not seeing any evidence that the US has done anything yet. The US Government is probably still brainstorming possible courses of action. If these documents really are a matter of national security, I'd imagine they'd have the authorization to take whatever action they needed under The Patriot Act and/or similar laws.
craz3d already said this, but to sort of repeat it:
how?
The is the Internet age. There are hundreds of mirrors of the wikileaks content, there's no single point of failure, and there's no way they're putting the toothpaste back in the tube.
Additionally, are you familiar with the insurance.aes256 file? It's a 1.4gb encrypted file, and is reportedly the unredacted contents of *everything*. Wikileaks has indicated that they'll release the decryption key if they reach a state where they can no longer function. That's basically blackmail (or extortion?), but does the US want to risk it?