WASHINGTON—The Pentagon has concluded that computer sabotage coming from another country can constitute an act of war, a finding that for the first time opens the door for the U.S. to respond using traditional military force.

The Pentagon's first formal cyber strategy, unclassified portions of which are expected to become public next month, represents an early attempt to grapple with a changing world in which a hacker could pose as significant a threat to U.S. nuclear reactors, subways or pipelines as a hostile country's military.

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