In its first months, the Trump administration has pivoted on trade, backing off from threats to overhaul the North American Free Trade Agreement and reversing Mr. Trump’s campaign pledge to label China a currency manipulator. Those changes are welcome, but in an interview last week with The Wall Street Journal, the president went further, saying that he might soften his trade stance in exchange for help with “the problem in North Korea.”

Mr. Trump may be ceding too much ground. In fact, he may have more leverage over China...