At the United Nations General Assembly on Tuesday, President Trump delivered an ultimatum to Iran — negotiate or face total military destruction — using language so unambiguous and violent that it ruptured the careful diplomatic grammar the forum has maintained since its founding. The moment raised a question that echoes far beyond any single bilateral dispute: whether the postwar architecture of multilateral restraint, built on the premise that nations choose words over weapons, still holds when the world's most powerful country abandons its conventions. History will record not only what was