Richard Nixon flew to Moscow in May 1972 to sign a pair of agreements with Leonid Brezhnev that tried—imperfectly—to put guardrails around the superpowers’ worst instincts. The logic seems quaint today: capping nuclear capability so the other guy isn't tempted to shoot first. It wasn't just about offensive weapons, however. The paperwork nodded to the "relationship between strategic offensive and defensive arms," a reminder that missile shields can make missiles feel "necessary." Today,