SHANGHAI - Over the last two months, Beijing has conveyed its concerns to North Korea about conducting a nuclear test. Yet on Feb. 12, China's neighbor detonated its third nuclear weapon -- smaller and more powerful than the two that preceded it. Foreign Minister Yang Jiechi said China was "strongly dissatisfied and resolutely opposed" to the test, but, as with North Korea's bad behavior in the past, will likely not follow with tougher action. China likely handles North Korea with kid gloves because it fears what would happen if the regime collapsed. If things turned bad, tens if not hundreds of thousands of refugees could flee across the border, destabilizing parts of northeastern China. North Korea's eventual reunification with South Korea might lead to a democratic U.S. ally with the potential for tens of thousands of U.S. and Korean troops stationed along China's border. And the millions of Korean-Chinese living in the border regions are one of China's most stable ethnic minorities; unlike the Tibetans and Uyghurs, they have not called for independence. If the Korean Peninsula were unified, this could change. This is part of the reason why China provides North Korea with presumably large amounts of aid as well as diplomatic cover at the United Nations. And what thanks does China get in return? Lies, insults, and provocations. On Jan. 22, after the U.N. Security Council unanimously passed Resolution 2087 as a response to Pyongyang's December "satellite" launch, North Korea responded by announcing that the six-party talks over its nuclear program have ceased to exist. Since 2003, China has worked hard to bring North Korea to those talks, asking for it to commit to nuclear abandonment while assuring it with development and security aid. China tried to water down the sanctions; instead of being grateful, Pyongyang hinted that some major powers had been manipulated by the United States. North Korea's threatening behavior, meanwhile, has made the region less stable. By firing artillery at South Korea's Yeonpyeong Island in 2010, killing four people, North Korea has pushed the envelope too far, undermining China's interests in keeping Northeast Asia stable. And now, this third nuclear test not only discredits Chinese diplomacy, but also provides a ready excuse for the United States to expand its military presence in the region. In his State of the Union address Tuesday, U.S. President Barack Obama vowed to boost America's missile-defense efforts in Asia -- U.S. technology that China doesn't welcome. Let's face it: China has reached a point where it needs to cut its losses and cut North Korea loose. China can't force North Korea to change its behavior simply by political means. North Korea is one of the most isolated states in the world, and its independence is a point of pride. While the regime survives due to its extraordinary resilience, China's economic help is an extremely important external source of sustenance. China's trade with North Korea rose to $3.1 billion in the first half of 2012, a rise of 24.7 percent; the 2011 trade figures of $5.7 billion represent a 62.4-percent gain over 2010. Beijing has also provided Pyongyang with aid in the form of energy, fertilizer, and other assistance. So, what is to be done? Through the Security Council, China should vote for tougher sanctions, while at the same time reducing aid and trade with its erstwhile ally. By acting multilaterally to curb North Korea, China could also strengthen its nonproliferation partnership with the United States and other countries in the region, fostering a more balanced U.S. Asian policy. China is dedicated to peace in East Asia. By pursuing its own national interest, China has also provided regional public goods, and has prevented a humanitarian disaster in North Korea. But Pyongyang's search for an independent deterrent indicates that it doesn't wish to put its security in Beijing's hands. So why should China continue to prop up this embarrassing maverick? The loss of this "ally" would be little felt in Beijing. China's view of its security interests has been much broadened over the last few decades, and with relations with Taiwan fast improving, North Korea's value as a security buffer has much diminished. And in an age where global public opinion matters more than ever, the benefits of association with Pyongyang's mistaken line outweigh the costs. At this stage, it's unlikely that North Korea's pattern of provocations will end. North Korea's repeated challenges have, time and again, made it a liability rather than an asset. Because it disrespected and hurt China's national interests, North Korea must pay a severe price.