The United States military has explicitly stated that it has the right to retaliate with military force against a cyber-attack.

In a 12-page report sent to Congress, the Department of Defense (DoD) said the military can launch a physical attack in the case of a cyber-attack against its systems. The threat of military action would act as a deterrent to people who think they can carry out “significant cyber-attacks directed against the US economy, government or military”, the Pentagon wrote in the report, which appears to be an update to the cyber-strategy plan released over the summer.

via eWeek, continued here.