A new policy document revealed United States's International Strategy for Cyberspace that deals with military and defence matters relating to cyber-attacks. The policy outlines several key measures the US would take to protect itself or its allies n the case of a cyberspace emergency:
States have an inherent right to self-defense that may be triggered by certain aggressive acts in cyberspace… 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.
This implies that should an "aggressive act" occur through cyberspace the United States will use its military forces to respond to the threat, as it would if a physical attack occurred on American or allied soil.
The strategy also emphasized that "The United States will ensure that the risks associated with attacking or exploiting our networks vastly outweigh the potential benefits" --hackers, be warned.
Here's the full document embedded below:
Refernce: International Strategy for Cyberspace (PDF)