Someday this White House will face a sudden, immediate and severe foreign-policy crisis. It’s almost a miracle it hasn’t happened already. George W. Bush had 9/11 less than eight months into his tenure; John F. Kennedy had the Bay of Pigs three months into his presidency and the Cuban missile crisis the following year. In two years Donald Trump has faced some turbulence, but not a full-fledged crisis.

Such good fortune won’t continue forever. I sometimes ask past and present officials of this administration their thoughts...