BEIJING — Shen Zhihua, a Chinese historian known for his groundbreaking research on the Korean War, has urged Beijing to rethink its longstanding support for North Korea. Mr. Shen made his case in a speech last month that has ignited widespread discussion in China, reflecting growing debate about how tough the government should be on North Korea. Here are some excerpts:

“Chairman Mao said long ago that who is our friend and who is our enemy is the question of first importance in a revolution. Getting to grips with that is also of the first importance in the foreign policy situation of northeast Asia. Just who is our friend, and who is our enemy? If you can’t distinguish between friend and foe, how can you fight and whom do you fight? Of course, friends can also have conflicts, and sometimes there’s also compromise and cooperation with enemies.

“If we look at North Korea and South Korea, who is a friend of China and who is an enemy? Outwardly, China and North Korea are allies, while the United States and Japan support South Korea against North Korea. That’s a legacy of the Cold War. But I believe that after decades of contention and shifts in the international landscape, there’s long been a fundamental transformation. My basic conclusion is judging by the current situation, North Korea is China’s latent enemy and South Korea could be China’s friend.

“To call North Korea a latent enemy of China means that, for now, this still hasn’t come to the fore. Diplomatically, when leaders of the two countries talk to each other, they don’t use particularly hostile rhetoric. But that doesn’t count. Don’t look at the rhetoric. Look at fundamental interests! Look at whether the fundamental interests of China and North Korea are aligned and consistent. Speaking in light of my own research into the history of the Chinese-North Korean relationship, China and North Korea really were friends and allies in the past. That was when the relationship was a special friendship created by Mao Zedong, Kim Il-sung and other senior Chinese and North Korean leaders.

“When China and South Korea established diplomatic relations in 1992, that totally destroyed the political basis of the Chinese-North Korean relationship. ... By 1992, at the end of the Cold War, the Chinese-North Korean relationship and alliance created by the previous generation no longer existed. Practically speaking, everything had changed in the relationship. In foreign policy, economics, politics, everything, the interests of China and North Korea had diverged, and the basis for an alliance had disintegrated. The treaty of alliance between China and North Korea became a piece of scrap paper. At that time the Chinese-North Korean relationship became an ordinary, normal relationship between states. But this normal relationship quickly and quietly turned toward hostility, and that was because North Korea launched its nuclear strategy.

“The root cause of the ever-worsening crisis on the Korean Peninsula is that North Korea has gone nuclear and is constantly holding nuclear tests, and that’s also the fundamental cause of instability on China’s periphery. But North Korea has been doing this for the sake of its fundamental interests. So, putting it objectively, the fundamental interests of China and North Korea are at odds. China’s fundamental interest lies in achieving stability on its borders and developing outward. But since North Korea acquired nuclear weapons, that periphery has never been stable, so inevitably Chinese and North Korean interests are at odds. The spokespeople for our Foreign Ministry claim that the North Korean nuclear crisis was triggered by antagonism between the United States and North Korea, and that’s entirely understandable as diplomatic language. But, as scholars, we must see clearly that North Korea’s shift to a policy of holding nuclear weapons was triggered by the shifts in its relationship with China.

“We must see clearly that China and North Korea are no longer brothers in arms, and in the short term there’s no possibility of an improvement in Chinese-North Korean relations. The situation now is that each time North Korea stages a nuclear test, the United States increases its military forces in northeast Asia, sending in drones or an aircraft carrier or holding military exercises. And then the military pressure from the U.S. leads North Korea to stage another nuclear test. You stage a test, he adds troops and it keeps escalating. The outcome? The real pressure is felt by China and South Korea, and the ones who ultimately bear the brunt are China and South Korea. ... So the upshot of North Korea stirring up trouble is more pressure and threats on China. Stepping back, if a Korean nuclear bomb explodes, who’ll be the victim of the nuclear leakage and fallout? That would be China and South Korea. Japan is separated by a sea, and the United States is separated by the Pacific Ocean.

“You shouldn’t do what your enemies want you to do, so I’ve been really disgusted by how China has handled Thaad [the Terminal High Altitude Area Defense antiballistic missile system being deployed in South Korea]. I have no idea whose idea that was, but what you’ve done is stir up such a ruckus that South Korean shops have had to close, and you’ve smashed up here, smashed up there. ... Putting it one way, you’ve got no foreign policy smarts. You’ve done exactly what your enemies would like you to do, and you’ve pushed South Korea into an iron triangle with the United States and Japan. Putting it another way, are you a civilized great power or not? Aren’t you a civilized ancient country, so why stoop to this? How do you want neighboring countries to view China? You just know how to pick on a company to blow off steam. You’re not using your brain. Isn’t there a mite of intelligence? By doing this we’ve alienated public opinion in South Korea, and in dealing with a democracy, the most important thing is to win over public sentiment and opinion. ... What we’ve been doing is just what the Americans and North Koreans want most of all. The North Koreans are also overjoyed, because the result of all this uproar over Thaad is that Chinese-South Korean relations have ruptured.”