But the recent nuclear threats by Kim Jong Un have raised tension to a new level. "This is really a crazy leadership" in North Korea, said Chu Shulong, an international relations professor at Tsinghua University in Beijing. "So dangerous."
Chu has long argued that China has been "too soft, too weak on North Korea" and should be tougher. "We need to do more sanctions," Chu said. "Let them know that we are angry and cannot accept their actions."
Instead, until now, China has sought to draw North Korea close, providing aid and investment and urging it to follow China's lead in opening up for economic modernization. Aid to North Korea grew from a third of China's aid budget a decade ago to half of its now-larger aid budget, according to Scott Snyder, senior fellow for Korea studies at the Council on Foreign Relations.
During visits to China, Kim Jong Il was taken to cellphone and car factories, fiber-optic plants and other showcases of Chinese economic reform. In 2011, the countries agreed to create two special economic zones in North Korea.
The goal was to produce a common interest in stability and development, but it did not work out that way.
"The argument was that we shouldn't give up" on North Korea, Chu said. "A certain group said, 'Everyone knows North Korea is a bad guy, but to isolate it would put us in a more dangerous position, so we should try to change North Korea.' "
Today the relationship is limited. Military sales ended many years ago. China provides about 10,000 barrels of crude oil a day, according to a Reuters report based on Chinese customs data. That modest amount at today's high prices could add up to nearly $400 million a year. China did not provide any oil to North Korea in February, but that was also the case in February in the previous two years.