Trump has famously boasted of his new relationship with Kim. By now, the president has learned—or should have learned—that Kim rulers do not reciprocate friendly gestures. They perceive them as signs of weakness and then press the advantage. While Trump was toasting and complimenting the current Kim, his regime was increasing production of fissile material and continuing construction of missile facilities.

The only way to disarm North Korea is to give whoever is in charge—Kim or perhaps someone else—no choice but to give up weapons. That means, as a practical matter, imposing extremely high costs for stunts like the one he—or they—just pulled. The Wall Street Journal in late May reported that UN and U.S. sanctions cut the flow of international payments to Kim in half. That number could be reduced by, say, another 40 percent, with vigorous enforcement, including a blockade.