There's plenty of proof now that the United States takes terrorism very seriously, and it will take whatever measures necessary for protections and deterrence. Besides just real, tangible attacks, the U.S. revealed its "International Strategy for Cyberspace" (PDF) which outlines things much to do with the Internet.
Ars Technica picked up on a few interesting points regarding the U.S.'s policy on cyberspace with regards to national defense – particularly with regards to responses to hacking and attacks.
"States have an inherent right to self-defense that may be triggered by certain aggressive acts in cyberspace," detailed the document. "Certain hostile acts conducted through cyberspace could compel actions under the commitments we have with our military treaty partners. … When warranted, the United States will respond to hostile acts in cyberspace as we would any other threat to our country."
As is the strategy of deterrence, the policy said, "The United States will ensure that the risks associated with attacking or exploiting our networks vastly outweigh the potential benefits."
Unlike land, the Internet isn't anchored to one nation in particular – regardless of its origins. It will be interesting to see how other countries' international cyberspace policies mesh up against this one.