WASHINGTON — As a presidential candidate, Donald J. Trump vowed to refill the cells of the Guantánamo Bay prison and said American terrorism suspects should be sent there for military prosecution. He called for targeting mosques for surveillance, escalating airstrikes aimed at terrorists and taking out their civilian family members, and bringing backwaterboarding and a “hell of a lot worse” — not only because “torture works,” but because even “if it doesn’t work, they deserve it anyway.” It is hard to know how much of this stark vision for throwing off constraints on the exercise of national...