President Trump has given China six months to prove that it is committed to preventing a nuclear-armed North Korea, and it seems his tolerance for China’s dithering has finally reached its limit. Now that President Xi Jinping has established that his government is unwilling to engage in a meaningful crackdown on its neighbor, the era of using carrots like improving trade relations to coax China into helping solve the “North Korea problem” has ended. It is now time for the stick.