They contain loose talk and confident predictions of the end of the dynasty that has ruled North Korea for 65 years. Those discussions were fueled by a rash of previously undisclosed defections of ranking North Korean diplomats, who secretly sought refuge in the South.

But they were also influenced by a remarkable period of turmoil inside North Korea, including an economic crisis set off by the government’s failed effort to revalue the currency and sketchy intelligence suggesting that the North’s military might not abide the rise of Mr. Kim’s son Kim Jong-un, who was recently made a four-star general despite having no military experience.

The cables reveal that in private, the Chinese, long seen as North Korea’s last protectors against the West, occasionally provide the Obama administration with colorful assessments of the state of play in North Korea. Chinese officials themselves sometimes even laugh about the frustrations of dealing with North Korean paranoia. In April 2009, just before a North Korean nuclear test, He Yafei, the Chinese vice foreign minister, told American officials at a lunch that the country wanted direct talks with the United States and to get them was acting like a "spoiled child" to get the attention of the "adult."

When James B. Steinberg, the deputy secretary of state, sat down in September 2009 with one of China’s most powerful officials, Dai Bingguo, state councilor for foreign affairs, Mr. Dai joked that in a recent visit to North Korea he “did not dare” to be too candid with the ailing and mercurial North Korean leader. But the Chinese official reported that although Kim Jong-il had apparently suffered a stroke and had obviously lost weight, he still had a “sharp mind” and retained his reputation among Chinese officials as “quite a good drinker.” (Mr. Kim apparently assured Mr. Dai during a two-hour conversation in Pyongyang, the capital, that his infirmities had not forced him to give up alcohol.)

But reliable intelligence about Mr. Kim’s drinking habits, it turns out, does not extend to his nuclear program, about which even the Chinese seem to be in the dark.

On May 13, 2009, as American satellites showed unusual activity at North Korea’s nuclear test site, officials in Beijing said they were “unsure” that North Korean “threats of another nuclear test were serious.” As it turns out, the North Koreans detonated a test bomb just days later.

Soon after, Chinese officials predicted that negotiations intended to pressure the North to disarm would be “shelved for a few months.” They have never resumed.