Another North Korean nuclear test, another round of demands that China bring Pyongyang to heel. Said U.S. Secretary of State John Kerry: Beijing's policy "has not worked and we cannot continue business as usual." Alas, his approach will encourage China to dismiss Washington's wishes.

North Korea may be the most vexing problem for the United States and its allies. Three successive U.S. presidents have insisted that the North simply cannot, must not, develop nuclear weapons. Yet it has. So attention naturally shifts toward China, which joined Washington in criticizing the latest blast. China is the North's most important investor and provides it with substantial energy and food assistance. Beijing also has protected North Korea by weakening past U.N. sanctions and enforcing those imposed with less than due diligence. If only China would get tough, runs the argument, Pyongyang would have to give way.

Alas, Chinese intervention is not the panacea many appear to believe. Contrary to common belief in Washington, the U.S. cannot dictate to China. Threats are only likely to make the Chinese leadership more recalcitrant.