China has the frustrating tendency to measure its willingness to help by discrete policy measures—a new sanction or statement—rather than by whether these measures have any material effect. President Trump’s frustration—“so much for China working with us”—is warranted. However, the problem he faces is deeper. In addition to their reticence, the Chinese have repeatedly stated that the purpose of pressure is not to bring North Korea to its knees, but to restart talks.

The current Chinese proposal for getting there starts with an unpalatable confidence-building measure that has come to be called “suspension-for-suspension.” North Korea would do little more than declare a moratorium on its nuclear and missile tests while the U.S. would suspend its annual military exercises with South Korea. Such a deal looks like straight extortion. But it is designed as a prelude to talks that would consider both American interests in denuclearization and North Korea’s proposal for a peace regime that would replace the Korean War armistice.

If talks don’t follow closely on the “suspension-for-suspension” trade, the U.S. will have paused military exercises for little more than a pause in Kim Jong Un’s march to a deliverable nuclear weapon.

The administration is thus searching around for leverage that would assure North Korea reconsiders its current course of action. Given limited military options, the administration has recently opted for secondary sanctions on Chinese entities doing business with North Korea, including a regional bank that is suspected of handling North Korean accounts. Such secondary sanctions would appear to make sense, and I support them. China has been enabling North Korea for long enough. By my calculations China does indeed account for about 90 percent of the country’s trade, although there is no evidence that bilateral trade has grown “40% in the first quarter” as another presidential tweet recently claimed. If the U.S. can bite into China-North Korea trade by dissuading larger Chinese firms from taking the risk of exposure to the country, then Kim Jong Un might well have miscalculated. Indeed, the country could be vulnerable to exactly the kind of foreign-exchange constraints that drove Iran to the bargaining table over its own nuclear program.

However, there is a catch. China strenuously opposes such measures and can easily dilute them by permitting other commercial trade and investment to proceed apace. Moreover, the administration has chosen the precise moment when it needs Beijing’s help to throw a succession of sharp elbows in China’s direction: a much-needed arms sale to Taiwan, signals that some trade enforcement actions are coming, and perhaps most concerning for the Chinese, a new freedom of navigation operation (FONOP in the lingo) in the contested South China Sea. Both the Taiwan and South China Sea actions might be warranted, yet are seen by China as touching on core sovereignty issues.