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Kim Jong-un’s mid-July shake-up of his military top brass occasioned a spate of media speculation about a transformation in North Korea. Yet Kim’s abrupt move only showed he was firmly enough in charge to fire the general whom his father had promoted to chief of staff. From what little we know about North Korea, it did not portend dramatic economic reform.

Potentially more significant, a widely anticipated third nuclear test by the North has not taken place. Preparations for such a test were under way when Kim’s father died. Kim’s restraint may signal that he wants to reengage with the United States, Japan and South Korea. That in turn could foster the more tranquil external environment he needs to turn his full attention to improving his...