Faced with a fresh barrage of threatening rhetoric by North Korea, its fourth nuclear test (January 6), and its subsequent successful launch of a ballistic missile capable of reaching the mainland United States, on March 31 President Barack Obama advocated closer security ties among America’s chief allies in the Far East. More significantly, he also urged increased cooperation with China to discourage Pyongyang.

As world leaders gathered for the fourth Nuclear Security Summit in Washington, Obama first  met with Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-hye. He then had a meeting with China’s President Xi Jinping at which both leaders urged North Korea (DPRK) to give up its nuclear arsenal. Xi also agreed to fully implement the latest economic sanctions against the regime of Kim Jong-un which were imposed by the United Nations Security Council (UNSC) on March 2. The wording of that resolution was stronger than initially expected, primarily due to China’s displeasure at Pyongyang’s tendency to take Beijing’s support for granted even when its actions run contrary to China’s strategic interests.

China’s longstanding priorities of “no war, no instability and no nukes” on the Korean Peninsula—in that order of priority—have produced ambiguous policies over the past decade. Two influential power centers in Beijing, the People’s Liberation Army General Staff Department and the International Department of_the_Communist_Party, continue to regard North Korea as an important geopolitical buffer between China and South Korea. They see the stability of the DPRK regime as more important than its compliance with non-proliferation strictures. Following North Korea’s second nuclear test (June 2009) China initially supported a sharply worded UN Security Council resolution, but in October of that year reversed its approach and effectively became North Korea’s protector and enabler, with former prime minister Wen Jiabao saying it was necessary to “put all our efforts without fail to boost peace and stability in Northeast Asia.”

The pendulum swung back again after Kim Jong-un succeeded his father Kim Jong-il (December 2011) and defiantly conducted DPRK’s third nuclear test in February 2013, only weeks after Xi Jinping became China’s “Paramount Leader.” Xi responded by supporting a tough UNSC resolution and for the first time implementing actual sanctions. Six months later, at the bilateral summit in California, he told Obama that they were “the same in their positions and objectives.” Nevertheless, to this day the Chinese leadership has continued to stress the need for peace and stability before denuclearization. At the same time, as CFR’s Korea expect Scott A. Snyder has noted, Kim Jong-un has thus far had good reason to take Beijing’s economic support for granted:

He knows that China’s main objectives are to prevent instability and war on the peninsula, so he may judge that China may be willing to acquiesce to a nuclear North Korea to avoid the greater dangers of potential military conflict. Likewise, Kim can capitalize on Chinese geostrategic fears: For Beijing, the only thing worse than a defiant North Korean buffer is a shared border with a unified peninsula under Seoul’s control, tied to the alliance with Washington.

There is in my opinion a neglected facet of China’s Korean strategy which may explain Beijing’s current policy and help us predict its future course. Ten months ago I wrote here that the South China Sea was fast becoming one of the key geopolitical battlegrounds of our time, because China’s multibillion-dollar island-building campaign has suddenly altered the strategic equation in the region.

Since then China has conducted a rapid program of militarization of three of those new islands, building deepwater port facilities and long airstrips, bringing fighter jets, and installing powerful radar systems, anti-ship and surface-to-air missile batteries. This has “strengthened China’s disputed claim to the entirety of the South China Sea, home to some of the world’s busiest shipping lanes.” The buildup has “challenged the military status quo in the Western Pacific since the end of World War II, bringing China closer to its goal of establishing a security buffer extending far from its coast—a dream of Chinese strategists since the Korean War.”

In view of this audacious geopolitical challenge, Obama’s call for increased cooperation with China to discourage Pyongyang’s nuclear ambitions must be music to Xi Jinping’s ears. It stands to reason that Beijing will agree to cooperate in chastising Kim Jong-un—for as long as this does not threaten the DPRK regime’s internal stability—in return for America’s tacit acceptance of China’s strategic fait accompli in the South China Sea.

It is an even bet that U.S. policymakers will consider this as an unpleasant but necessary bargain. Japan and South Korea are primarily interested in containing the threat from North Korea, and their clout in Washington is far greater than that of the Philippines, Vietnam, and other claimants to the “Asian Mediterranean.” On the grand Asian chessboard, Kim’s recklessness in one area has helped strengthen China’s position in another.