The Pentagon has concluded that the act of sabotaging U.S. computers by another country may constitute an act of war against the United States. This conclusion opens the door for the United States to respond to such cyber sabotage by using traditional military force. The Pentagon’s first formal cyber strategy, unclassified portions of which will become public next month, is an early attempt to come to terms with a changing world in which hackers could pose as significant a threat to U.S. nuclear reactors, electric grid, or pipelines as a hostile country’s military.