Presidents should scrupulously avoid red lines and ultimatums unless they are fully committed to enforcing them militarily. Once a president cocks the pistol, he or she must be ready to fire it. Obama’s failure to enforce his own red line with respect to Assad’s use of chemical weapons against the Syrian opposition echoed around the world—but especially in Moscow, Beijing, Tehran, and Pyongyang. Trump’s repeated threats of massive military attacks on North Korea and Iran by the end of 2019 increasingly were viewed by adversaries as hollow bluffs.
