Violence as savage and public as the massacre that took place at a Chinese train station on Saturday shocks the chemistry of a country in a way that years of more remote, simmering conflict do not. Acts of such spectacular violence exert unpredictable forces on the public and on the leaders who are charged with protecting it, transforming judgments of when and how to use force and decisions about what can be sacrificed in the name of security, as well as the definitions of citizenship, patriotism, and innocence. Rarely do they leave anyone better off than they were before.

When eight assailants armed with foot-long sabers set upon men and women in the southwestern city of Kunming, killing at least twenty-nine people and injuring a hundred and forty-three, they struck in a place and a manner that nobody in China had anticipated. For all its epic history of bloodshed, the People’s Republic is unaccustomed to this kind of threat against citizens going about their daily lives, and, by day’s end, the attack was seared into public consciousness in a way that, since 9/11, has become customary for these moments around the world: it is the 3/1 incident. A message in wide circulation declared, “We are all Kunmingers.”

Chinese authorities say the attack was “orchestrated by Xinjiang separatists.” Xinjiang is the homeland of the Uighurs, one of China’s fifty-six officially recognized ethnic groups. Uighurs have been in contact with China for two millennia, but the region was given the name Xinjiang—Mandarin for “new frontier”—in 1884, when it was declared a province of the Chinese Empire for the first time. In 1955, it was converted into the largest of China’s five autonomous regions for ethnic minorities (which include Tibet and Inner Mongolia) and it maintained a fitful relationship with Beijing.

After the train-station attack, state media reported that an Islamic flag was among the items found at the scene, but no group has claimed responsibility. Over the years, militant Uighurs have formed various organizations, including the East Turkestan Islamic Movement (E.T.I.M.), which the U.S. Treasury Department classified as a terrorist organization in 2002, during the period of heightened U.S.-Chinese antiterrorism coöperation that followed September 11th. The Treasury Department later identified the E.T.I.M. leader Abdul Haq as a member of Al Qaeda’s leadership council; he is believed to have been killed in a U.S. drone strike, in 2010. The Chinese government blamed E.T.I.M. for a suicide car crash in Tiananmen Square last October, which killed five people, but the U.S. stopped short of drawing that connection.

Within hours of this attack, President Xi Jinping called for “an all-out effort to punish the terrorists.” As I wrote last year, the pressure posed by ethnic unrest is the biggest story on the Chinese horizon, and that struggle—the pressure from below, and the response it will bring—just moved into the foreground. In ways that may run deeper than even the attackers intended, the Kunming massacre is likely to harden Chinese leaders against critical opposition. For a generation of senior Community Party members, the attack is a sensational confirmation of what has become the most neuralgic issue of their time: the sense that the greatest threat to the country as they know it is the loss of territory. Shortly after taking office, in November, 2012, Xi Jinping, in a speech to Party members, asked, “Why did the Soviet Communist Party collapse? An important reason was that their ideals and convictions wavered. Eventually, all it took was a quiet word from Gorbachev to declare the dissolution of the Soviet Communist Party, and the great party was gone. In the end nobody was man enough to come out and resist.”

On its face, Xi’s “man enough” speech was regarded as a case against Western-style democratization, which, of course, it was. But that is a narrow reading. For much of the past decade, an emerging argument in Chinese policy and scholarly circles has come to see the failure of the Soviet Union as a failure to manage ethnic unrest.

In 1986, when protesters in Kazakhstan took to the streets, declaring, “Kazakhstan belongs to Kazakhs,” Mikhail Gorbachev sent in troops, but he also made efforts to appease the rioters by appointing a Kazakh apparatchik and by relenting on unpopular laws about language. Other ethnic groups mounted their own rebellions. Ma Rong, a well-connected sociologist at Peking University, later wrote that this chain of events “reminded the P.R.C. leaders of the political risk in managing ethnic relations, and made them very cautious.” Writing in an academic journal, in 2007, Ma suggested that “the former Soviet Union took a great risk by handling its nationality/ethnicity issues the way it did.” The Soviets, he argued, wrongly assumed that Communism would bind their ethnicities together, but, in fact, the “nation was at risk of disintegrating if the ideological linkage among the ethnic groups collapsed.” When Chinese leaders say, as they often do, that “stability in Xinjiang” or “stability in Tibet” concerns the “stability of the country,” they mean it.

In China today, the ties between ethnic groups are rooted not in Communism but, for lack of a better word, in “G.D.P.-ism”—faith in economic growth and the push for prosperity. But that is a fragile bargain. Militant Uighurs are motivated largely by resentment of their relationship to Han Chinese. Xinjiang’s Uighur population has dropped from ninety-five per cent, in the early twentieth century, to forty per cent, in 2008, thanks to an explicit migration policy intended to bind the country more tightly. On the ground, the development policy has created vast new infrastructure and economic activity, but, crucially, it has also accentuated the socioeconomic gaps between Hans and Uighurs. In Xinjiang today, Hans hold more than thirty five per cent of the region’s the high-income jobs, while Uighurs hold thirteen per cent. The ratio is widening by the year, fuelled by, and creating, even more resentment and suspicion. The events of 3/1 will make that worse.

Above: Police officers stand guard at the Changsha Railway Station, in Changsha city. Photograph by ImageChina/AP.