John Bolton will assume office Monday with his first controversy as President Trump’s national security adviser awaiting him. Six weeks ago, he outlined his advocacy of an attack on North Korea in a Wall Street Journal op-ed titled “The Legal Case for Striking North Korea First.”

“Given the gaps in U.S. intelligence about North Korea,” he wrote, “we should not wait until the very last minute” to stage what he called a pre-emptive attack.

Mr. Bolton’s legal analysis is flawed and his strategic logic is dangerous. As he did before the 2003 Iraq war, he is obscuring the important distinction between preventive and pre-emptive attacks. Under rules of international law based on Daniel Webster’s interpretation of the Caroline case in 1837, a pre-emptive attack can be legal, but only if an adversary’s attack is imminent and unavoidable — when a need for self-defense is “instant” and “overwhelming.”

For example, if America had intelligence that North Korea had alerted military forces and was fueling long-range missiles on their launchpads or rolling out missile launcher vehicles, the United States could reasonably assume an attack was imminent and unavoidable and could legally launch a pre-emptive strike in what international lawyers call “anticipatory self-defense.”