A year into Donald J. Trump’s presidency, his defense secretary, Jim Mattis, sent the new commander-in-chief a startling recommendation: with nations around the world threatening to use cyberweapons to bring down America’s power grids, cell-phone networks, and water supplies, Trump should declare he was ready to take extraordinary steps to protect the country. If any nation hit America’s critical infrastructure with a devastating strike, even a non-nuclear one, it should be forewarned that the United States might reach for a nuclear weapon in response.
