Thirty minutes. That’s about how long it would take a nuclear-tipped intercontinental ballistic missile (ICBM) launched from North Korea to reach Los Angeles. With the powers in Pyongyang working doggedly toward making this possible—building an ICBM and shrinking a nuke to fit on it—analysts now predict that Kim Jong Un will have the capability before Donald Trump completes one four-year term. About which the president has tweeted, simply, “It won’t happen!” Though given to reckless oaths, Trump is not in this case saying anything that departs significantly from the past half century of futile American policy toward North Korea. Preventing...