President Obama imagined he could end his second term with an arms-control detente with Iran the way Ronald Reagan did with the Soviet Union. It looks instead that his nuclear deal has inspired Iran toward new military aggression and greater anti-American hostility.

The U.S. and United Nations both say Iran is already violating U.N. resolutions that bar Iran from testing ballistic missiles. Iran has conducted two ballistic-missile tests since the nuclear deal was signed in July, most recently in November. The missiles seem...