China is North Korea's most important ally, biggest trading partner, and main source of food, arms, and fuel. China has helped sustain Kim Jong-Il's regime and opposed harsh international economic sanctions in the hope of avoiding regime collapse and an uncontrolled influx of refugees across its 800-mile border with North Korea. After Pyongyang tested a nuclear weapon in October 2006, experts say that China has reconsidered the nature of its alliance to include both pressure and inducements. North Korea's second nuclear test in May 2009 further complicated its relationship with China, which has played a central role in the Six-Party Talks, the multilateral framework aimed at denuclearizing North Korea. CFR's Scott Snyder and See-won Byun of the Asia Foundation argue the nuclear tests highlight the tensions (PDF) between China's "emerging role as a global actor with increasing international responsibilities and prestige and a commitment to North Korea as an ally with whom China shares longstanding historical and ideological ties." Beijing continues to have more leverage over Pyongyang than any other nation, say some analysts. The economic leverage in particular, some point out,  has only grown as a result of North Korea's declining relations with South Korea and the international community. But most experts agree that Beijing is unlikely to exercise its leverage given its concerns regarding regional stability and the uncertainty surrounding regime succession in North Korea.

China has supported North Korea ever since Chinese fighters flooded onto the Korean peninsula to fight for their comrades in the Democratic People's Republic of Korea (DPRK) in 1950. Since the Korean War divided the peninsula between the North and South, China has lent political and economic backing to North Korea's leaders: Kim Il Sung and his son and successor, Kim Jong-Il.

In recent years, China has been one of the authoritarian regime's few allies. But this long-standing relationship suffered a strain when Pyongyang tested a nuclear weapon in October 2006 and China agreed to UN Security Council Resolution 1718, which imposed sanctions on Pyongyang. By signing off on this resolution--as well as earlier UN sanctions that followed the DPRK's July 2006 missile tests--Beijing departed from its traditional relationship with North Korea, changing from a tone of diplomacy to one of punishment. China also agreed to stricter sanctions after Pyongyang's second nuclear test in May 2009. Alan Romberg, a former U.S. State Department official with the Henry L. Stimson Center in Washington told TIME, "Pyongyang has spit in the [People's Republic of China's] eye." Jonathan D. Pollack, an East Asia expert at the Naval War College, described the DPRK's 2006 tests also as "jarring" to China's diplomatic effort to compel North Korea to the Six-Party Talks. He says Kim Jong-Il was effectively telling Beijing, "You cannot tell us what to do and we cannot be taken for granted." Despite their long alliance, experts say Beijing does not control Pyongyang. "In general, Americans tend to overestimate the influence China has over North Korea," says Daniel Pinkston, a Northeast Asia expert at the International Crisis Group.

At the same time, China has too much at stake in North Korea to halt or withdraw its support entirely. "The idea that the Chinese would turn their backs on the North Koreans is clearly wrong," says CFR Senior Fellow Adam Segal. Beijing only agreed to UN Resolution 1718 after revisions that removed requirements for tough economic sanctions beyond those targeting luxury goods, and China's trade with North Korea has continued to increase. Bilateral trade between China and North Korea reached $2.79 billion in 2008, up 41.3 percent compared to 2007. The Chinese are "doing just what they have to do and no more" in terms of punishing North Korea, says Selig S. Harrison, Asia program director at the Center for International Policy. He says the countries will not jeopardize their mutually beneficial economic relationship. The economic effect of UN Resolution 1874, passed after the second nuclear test in 2009, is also not likely to be great unless China cooperates extensively and goes beyond the requirements of the resolution, says a July 2009 report (PDF) from the U.S. Congressional Research Service.

Pyongyang is economically dependent on China, which provides most of its food and energy supplies. Nicholas Eberstadt, a consultant at the World Bank, says that since the early 1990s, China has served as North Korea's chief food supplier and has accounted for nearly 90 percent of its energy imports. By some estimates, China provides 80 percent of North Korea's consumer goods and 45 percent of its food. North Korea's economic dependence on China is rapidly increasing, as indicated by a significant trade imbalance. Snyder notes that in 2008, Chinese imports amounted to $2.03 billion, while exports to China including coal and iron ore totaled $750 million. Some experts see the $1.25 billion trade deficit as an indirect Chinese subsidy given that North Korea cannot finance its trade deficit through borrowing.

China also provides aid directly to Pyongyang. "It is widely believed that Chinese food aid is channeled to the military," (PDF) reports the Congressional Research Service. That allows the World Food Program's food aid to be targeted at the general population "without risk that the military-first policy or regime stability would be undermined by foreign aid policies of other countries."

China is also a strong political ally. "As an authoritarian regime that reformed, they understand what Kim Jong-Il is most concerned with--survival," Segal says.

China's support for Pyongyang ensures a friendly nation on its northeastern border, as well as provides a buffer zone between China and democratic South Korea, which is home to around twenty-nine thousand U.S. troops and marines. This allows China to reduce its military deployment in its northeast and "focus more directly on the issue of Taiwanese independence," Shen Dingli of the Institute of International Studies at Fudan University in Shanghai writes in China Security (PDF). North Korea's allegiance is important to Beijing as a bulwark against U.S. military dominance of the region as well as against the rise of Japan's military.

China also gains economically from its association with North Korea; growing numbers of Chinese firms are investing in North Korea and gaining concessions like preferable trading terms and port operations.

"For the Chinese, stability and the avoidance of war are the top priorities," says Daniel Sneider, the associate director for research at Stanford's Asia-Pacific Research Center. "From that point of view, the North Koreans are a huge problem for them, because Pyongyang could trigger a war on its own." The specter of hundreds of thousands of North Korean refugees flooding into China is a huge worry for Beijing. "The Chinese are most concerned about the collapse of North Korea leading to chaos on the border," CFR's Segal says. If North Korea does provoke a war with the United States, China and South Korea would bear the brunt of any military confrontation on the Korean peninsula. Yet both those countries have been hesitant about pushing Pyongyang too hard, for fear of making Kim's regime collapse. The flow of refugees into China is already a problem: China has promised Pyongyang that it will repatriate North Koreans escaping across the border, but invites condemnation from human rights groups when sending them back to the DPRK. Jing-dong Yuan of the Center for Nonproliferation Studies in California says Beijing began its construction of a barbed wire fence along this border in 2006 for that reason.

Experts say China has also been ambiguous on the question of its commitment to intervene for the defense of North Korea in case of military conflict. The 1961 Sino-North Korean Treaty of Friendship, Cooperation and Mutual Assistance says China is obliged to defend North Korea against unprovoked aggression. But Jaewoo Choo, assistant professor of Chinese foreign policy at Kyung Hee University in South Korea, writes in Asian Survey that "China conceives itself to have the right to make an authoritative interpretation of the principle for intervention," (PDF) in the treaty.  As a result of changes in regional security in a post-Cold War world, he writes, "China now places more value on national interest, over alliances blinded by ideology." But, he argues, Chinese ambiguity deters others from taking military action against Pyongyang.

Beijing has been successful in bringing North Korean officials to the negotiating table at the Six-Party Talks many times. "It's clear that the Chinese have enormous leverage over North Korea in many respects," Sneider, of Stanford's Asia-Pacific Research Center, says. "But can China actually try to exercise that influence without destabilizing the regime? Probably not." Pinkston says that for all of North Korea's growing economic ties with China, Kim still makes up his own mind: "At the end of the day, China has little influence over the military decisions."

Also, China does not wish to use its leverage except for purposes consistent with its policy objectives and strategic interests, say experts. Choo writes, "After all, it is not about securing influence over North Korean affairs but is about peaceful management of the relationship with the intent to preserve the status quo of the peninsula." This CFR.org Crisis Guide offers an in-depth analysis of the dispute on the Korean peninsula.

The United States has pushed North Korea to verifiably and irreversibly give up its nuclear weapons program in return for aid and diplomatic benefits, and eventually normal diplomatic relations with Washington. Experts say Washington and Beijing have very different views on the issue. "Washington believes in using pressure to influence North Korea to change its behavior, while Chinese diplomats and scholars have a much more negative view of sanctions and pressure tactics," Pinkston says. "They tend to see public measures as humiliating and counterproductive."

However, China and the United do share common interests, including containing North Korea's nuclear program and preventing South Korea and Japan from going nuclear, say some experts. A regional partnership involving the United States and the countries of Northeast Asia, including China "remains the best vehicle ... for building stable relationships on and around the Korean peninsula," writes CFR Senior Fellow Sheila A. Smith.

"Everyone who deals with North Korea recognizes [it] as a very unstable actor," Sneider says. However, some experts say North Korea is acting assertively both in its relationship with China and on the larger world stage. "The North Koreans are developing a much more realist approach to their foreign policy," Pinkston says. "They're saying imbalances of power are dangerous and the United States has too much power--so by increasing their own power they're helping to balance out world stability. It's neorealism straight out of an international relations textbook."

And even though China may be angry at North Korea's nuclear brinkmanship, analysts say it will avoid moves that could cause a sudden collapse of the regime. Given the competition for influence in preparation for the eventual passing of a physically weak Kim Jong-Il, China may feel even more restrained from pressuring North Korea for fear of alienating a future power base.

But Asian military affairs expert Andrew Scobell writes, "No action by China should be ruled out where North Korea is concerned." According to Scobell, Beijing might stop propping up Pyongyang and allow North Korea to fail if it believed a unified Korea under Seoul would be more favorably disposed toward Beijing. A January 2008 report by the Center for Strategic and International Studies and the U.S. Institute of Peace, two Washington-based think tanks, says China has its own contingency plans (PDF) to dispatch troops to North Korea in case of instability. According to the report, the Chinese army could be sent into North Korea on missions to keep order if unrest triggers broader violence, including attacks on nuclear facilities in the North or South.

Beijing may find ways to cause North Korea discomfort, but Hayes describes China as "patient" and foresees Beijing undertaking long-term training of North Koreans in China to help stabilize the country. "The Chinese are thinking one hundred years ahead," he says. "China will conduct inside-out transformation of North Korea over the next twenty years." Andrei Lankov, associate professor at Kookmin University in Seoul, wrote in Foreign Affairs in March 2008 that China is tiring of pouring aid into the inefficient North Korean economy. "The Chinese government is promoting its own style of reform in Pyongyang: economic liberalization with limited, incremental political change," he writes. But he says China, so far, has failed and "North Korea's leaders are in no hurry to introduce any reforms."

Esther Pan and Carin Zissis contributed to this Backgrounder.