In his memoirs, former U.S. President George W. Bush recounts a story about North Korea and China. In October 2002, he invited China's then-president, Jiang Zemin, to his Texas ranch. North Korea was developing nuclear weapons, and Bush wanted China's help. According to Bush, Jiang told him that "North Korea was my problem, not his." China did nothing.
A few months later, Bush tried a different tack. He told Jiang in January 2003 that if North Korea's nuclear weapons program continued, the United States would not be able to stop Japan from developing its own nuclear arsenal. Still nothing.
A month later he warned China that if the problem was not solved diplomatically he would consider a military strike against North Korea. Only at that point did China react. Talks with North Korea were commenced, but the hermit kingdom continued its nuclear program and last month conducted its third nuclear test.