In the 1950s, soldiers from North Korea and China bled and died together to repulse Western forces in horrific fighting over control of the Korean peninsula. In the 1960s, they signed a friendship treaty promising to defend each other.

Over the decades that followed, the two countries that Mao Zedong called "as close as lips and teeth" forged bonds of friendship and common cause. Chinese equipment powered North Korean concrete and electricity plants. Trade raised mutual profits. And a sense of Communist fraternity wrested reconciliation from discord and curried solidarity against a Western order that both sides viewed as hostile.

After China detonated its first nuclear weapon in 1964, North Korean "great leader" Kim Il-sung even appealed to his brothers in Beijing to help him build his own atomic bomb.

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But for Chairman Mao, that was a step too far.

He rejected the request, and a similar one a decade later, according to an account in The Two Koreas: A Contemporary History, by Don Oberdorfer and Robert Carlin. "North Korea is a very small country," is how a Chinese official put it to the authors of the 2014 book – and China determined that North Korea simply did not need an atomic bomb.

Even a half-century ago, there were limits to what China would tolerate.

Now, with Pyongyang seemingly on the verge of possessing a nuclear weapon that it could affix to a missile to hit targets thousands of kilometres away, China is once again being forced to consider what it will countenance.

Still bound to North Korea through family ties between citizens on both sides of the border, by financial investments, and by a 1,400-kilometre-long shared frontier, China stands at the forefront of the world's tensions with Pyongyang. North Korea's preparations for a sixth nuclear test have brought extraordinary disquiet to northeast Asia, with Washington directing some of its most potent military assets to the region.

In mid-April, Chinese Foreign Minister Wang Yi warned that "one has the feeling that a conflict could break out at any moment," and Chinese president Xi Jinping spoke by telephone with Donald Trump on Monday, asking the U.S. leader to "avoid doing anything to worsen the tense situation on the peninsula." Instead, Mr. Trump warned in an interview with Reuters that diplomatic solutions are very difficult, and "there is a chance that we could end up having a major, major conflict with North Korea."

Against that backdrop, signs are emerging that an angry and anxious China is reconsidering its long-standing brotherhood with North Korea. It is increasingly fed up with the isolated regime that has spurned the world by chasing after ever-deadlier nuclear weapons – a startling reversal of sentiment from a country that was once the world's most reliable and sympathetic defender of North Korea.

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"People living here have a deep sense of fatigue," says Jin Qiangyi, director of the Centre for North and South Korea Studies at Yanbian University in China's northeastern city of Yanji. They "are growing tired of it all."

Nowhere is that more clear than along the mountains and rivers of the complicated dividing line between the two countries, where the risks grow frighteningly real. Isolated, sparsely populated, and far away from the country's modern centres of economic power, the frontier has long been the manifestation and wellspring of China's worries about North Korea. Here, the country led by a succession of dictators with outsized personalities is not a distant concept or object of late-night Western television mockery. Rather, it is a place that is plainly visible – and occasionally, quite literally, felt. At least twice, North Korean nuclear tests have shaken this area, with an earthquake-like jolt that rattled homes and nerves.

The flags of Russia, China and North Korea fly on a viewing tower on the border between the three countries in Hunchun, in China’s northeast Jilin province. GREG BAKER/AFP/GETTY IMAGES

TRISH McALASTER/THE GLOBE AND MAIL

Prof. Jin's cigarette-scented office sits just 200 kilometres from North Korea's Punggye-ri nuclear test site, where an atomic bomb may be ensconced deep below a granite mountain where five other nuclear devices have already been detonated. Satellite imagery suggests a sixth awaits nothing more than a final instruction from "Supreme Leader" Kim Jong-un before it is fired. Such a test would propel the region into a fraught unknown, after unprecedented efforts by China and the U.S. to pressure North Korea to abandon its pursuit of such deadly new firepower.

"The current state of things is more tense than it has ever been in the past," says Prof. Jin.

Ethnically Korean, he grew up in the area, regularly travels to Pyongyang, and is one of China's best-informed scholars on the topic of the Korean peninsula.

China has dismissed reports that it has sent 150,000 additional troops to the border. But the security presence is unmistakeable. A dozen uniformed men at an armed checkpoint, a short distance from the crossing to a sizable North Korean city, searched and detained this Globe and Mail reporter when I drove to the area last week. They allowed passage only after local police agreed to an escort to an overlook of the physical border – and on condition that no photos be taken and no interviews conducted. "Border restrictions are quite tight right now," one officer said, calling the area "sensitive."

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At the nearby Yanji airport, which doubles as a military air base, nearly a dozen fighter jets filled the hangars on a recent afternoon, and crews appeared to be preparing most of them for flight. Troops marched through the airport parking lot, and a local taxi driver said he had been warned by police: "Be careful when you speak with foreigners."

Crews prepare fighter jets at China’s Yanji airport amid elevated tensions with North Korea. NATHAN VANDERKLIPPE/THE GLOBE AND MAIL

A government document obtained by The Globe from the environmental-protection bureau in Dalian, near the western terminus of China's border with North Korea, underscores the acuteness of local fears. Issued a day before an expected nuclear test on April 15 – the test did not happen, although foreign governments are now bracing for another possible test on Tuesday – the document orders local leaders to maintain a state of high alert.

"Our city has entered emergency stand-by status as of today, and all relevant units need to organize emergency preparation operations," it says, specifically pointing to "the possible impact of and harm caused by a sudden incident in the North Korean nuclear and chemical environment." Local officials declined comment when reached by The Globe, but did not deny the document's existence. This week, another notice was issued in Dandong, which is also a border city, mandating the registration of Korean speakers who can be called on as interpreters in case of a "sudden foreign incident."

Given that the risks from North Korea are concentrated on the borderlands, the frontier has long loomed large in how Beijing thinks about Pyongyang. A North Korean nuclear test gone awry could produce a cloud of contamination that would waft across the border. Armed conflict or political chaos could send millions of refugees toward China. It could even set in motion reunification of the peninsula under South Korean leadership – an eventuality that would bring a key U.S. ally, backed by American military might, onto China's doorstep.

The potential for those outcomes has long shaped the contours of China's policies on North Korea. A succession of Chinese leaders have been adamant that Pyongyang must denuclearize. But they have been unwilling to cut North Korea off in a way that might risk the country's falling apart. That explains why trucks continue to move across the border today, and why local businessmen scoff at the notion that there is anything to worry about.

But across the region, people are also giving voice to a new thought, one that underlies a major change in the way that North Korea's powerful northern neighbour deals with the rogue nation: Enough is enough.

A propaganda poster from the 1950s shows Mao Zedong standing in front of a red flag with portraits of Stalin, Lenin, Engels and Marx. EVERETT COLLECTION

A North Korean propaganda image shows Kim Il-sung, right; his wife, Kim Jung-sook; and their son, Kim Jong-il. KCNA/REUTERS

Bonds of blood, now frayed

On the first day of October, 1950, Kim Il-sung dispatched a special airplane to Beijing with a message for Mao Zedong.

It contained a plea: Help.

Four months earlier, North Korea had invaded the South, taking Seoul. Now, a powerful counterattack was under way, and Mr. Kim was terrified at the prospect of defeat. "We shall not be able to survive relying merely on our own strength," Mr. Kim wrote.

Little more than two weeks later, on the same day that Pyongyang fell, Chinese forces secretly crossed into North Korea; less than three months later they would retake Pyongyang. By the time the fighting stopped, some 900,000 Chinese soldiers and 520,000 North Koreans were dead or injured – cementing a bond of blood that would be formalized in 1961 with the Sino-North Korean Mutual Aid and Co-operation Friendship Treaty, which committed China to coming to its neighbour's defence if the need arose.

Still today, the border areas make those ties tangible. For many ethnically Korean Chinese, North Korea is home to close family relatives. For Chinese traders and manufacturers who have built factories in North Korea, the country is a source of profit, as it is for local tour companies who organize groups of Chinese to gawk at life on the other side. A few living older Chinese fought during the Korean War, retaining memories of a shared experience of battle alongside North Korean troops.

Even here, however, it is increasingly clear that things are changing, with local anxieties reflecting a broader reality that Chinese sympathies for North Korea are running low. Cross-border family ties have weakened with time, as a generation once united fades away, replaced by generations born into a time that knows only a cleft geography; China's young people are more in tune with South Korean music and TV than with uncles and aunts in North Korea.

At the national level, too, decades of plans for massive development projects – not least, those involving long-term ambitions to create a Sino-Russo-Korean trading hub at North Korea's northeastern Rajin port – have fallen by the wayside. A giant friendship bridge spanning the border at the Chinese city of Dandong has gone unused despite its construction being largely finished in 2015.

Conflict, particularly if it involves detonation of nuclear weapons, could bring catastrophe here. A worried China has repeatedly called for calm. On Monday, President Xi spoke by phone with Mr. Trump, and asked the U.S. leader to "avoid doing anything to worsen the tense situation on the peninsula."

But the status quo – tensions that wax and wane but never disappear, even as North Korea builds ever-bigger bombs – has begun to draw criticism from unusual places. Last month, historian Shen Zhihua, one of China's foremost voices on the Cold War and Korean-peninsula issues, said in a landmark speech that it was time for academics to speak the truth rather than worry about angering those in power.

And the truth, he said, is that China's approach to the Korean peninsula no longer works. It is the trade-friendly South, not the isolated North, that should be China's friend, he said – and the nuclear-armed North, not the nuclear-free South, that should be considered an enemy.

"We must see that China and North Korea are not comrades, and in the short term, China-DPRK relations cannot be improved," he said, employing the name that North Korea uses for itself: the Democratic People's Republic of Korea.

He added: "Our goal should be to completely solve the Korean-peninsula issue, not to maintain the status quo."

A North Korean band performs for tourists at a hotel in Hunchun. GREG BAKER/AFP/GETTY IMAGES

Disgust with a brutal regime

In the melting pot of culture and cuisine that marks the Chinese side of the border, people say change is already obvious, starting with a rising disdain for the country across the river. "I have even thought that maybe there should be war," says a local businesswoman, who identifies herself only as Ms. Kim, when we speak inside a busy convenience store she runs in a Yanji residential district. "I have to say, I feel anger for Mr. Kim," she adds, referring to the man who has accelerated the nuclear-weapons program inaugurated by his grandfather, Kim Il-sung.

Her distaste springs from an unexpected concern: Her impoverished relatives in North Korea have suffered badly, and the gap between their economic outlook and Ms. Kim's own rising prospects grows wider by the year. "Their lives there are very unfair," she says. If war frees them from their current leadership, maybe it would "provide them justice and fairness."

Another Yanji man who has family working at the border tells me he is "angry" at Mr. Kim, citing his disgust with a regime that tortures people who are discovered and sent back after trying to flee to China.

When Adam Cathcart, who co-edits the Sino-NK website that closely follows developments between the two countries, travelled through the northeastern border regions in mid-April, he spoke with a Yanji bookseller who told him: "Well, if there's a war, maybe it won't be so bad. It will just mean South Korea controls the other side of the river."

Reality, it goes without saying, is far more complex. But the man's comment shows how "under the surface, things seem to be already shifting," says Mr. Cathcart, a lecturer at the University of Leeds.

Among those developments are signs that China is preparing for disaster. Last fall, a devastating flood swept down the Tumen River, which forms the northeastern boundary between North Korea, China and, for a short distance, Russia. China mounted a rapid evacuation response that "was a kind of dry run for them of a contingency scenario at the North Korean border," says Mr. Cathcart.

The Chinese People's Liberation Army, he adds, "is pretty serious about protecting that frontier, and I think they are fairly nervous and will need to move fast if things get out of hand."

Kim Jong-un and Xi Jinping. KCNA, REUTERS

Chinese skepticism toward its neighbour has built since North Korea's first nuclear test, in 2006, growing stronger under President Xi, who came to power in 2012. Unlike his predecessors, who welcomed North Korean leaders into the exclusive enclave for China's political elite at Zhongnanhai, Mr. Xi has never met Kim Jong-un.

"You could say that China, since Xi came to power, is increasingly fed up with North Korea," says Ye Fei, a political analyst at the Beijing research and advisory firm China Policy. He has scrutinized the changing tone emanating from Chinese political leaders and academics.

A sense of socialist solidarity once created bonds of friendship between the two countries, bonds that extended beyond any other political or economic considerations. "Under Xi, there is none of that," says Mr. Ye. "Xi is really trying to treat Kim the third as a juvenile hooligan."

Chinese scholars are even openly questioning the nature of the 1961 Sino-North Korean friendship treaty. It was signed at a very different time, and should no longer be seen as committing China to defend North Korea, says Lu Chao, director of the Border Study Institute at the Liaoning Academy of Social Sciences, a government think tank.

For China to abandon a military alliance would mark a major change, one that has not been officially discussed. But "frankly speaking, we are right now at the lowest point in the relationship between China and North Korea," says Mr. Lu. "China has continually opposed their nuclear development and tried to persuade them not to continue. But North Korea has completely ignored this."

Visitors look out toward Russia, left, and North Korea, right, at the viewing tower in Hunchun. GREG BAKER/AFP/GETTY IMAGES

Experimenting with sanctions

The question now: How far is China willing to go to bring its unruly neighbour to heel?

Chinese efforts to rein in North Korea have always taken place inside cautiously set limits, precisely to avoid provoking catastrophe.

Recent signs point to a much more concerted approach, particularly as Chinese leaders and commentators indicate that they are willing to make North Korea feel new pain if it detonates its sixth nuclear weapon.

"Going ahead with the nuclear test will constitute a serious provocation to the international community, and will bring the most serious sanctions, including oil," says Prof. Lu. Severing oil shipments to North Korea – which relies almost exclusively on imported crude, most of it from China – would paralyze the country's economy and military. But mention of this option in China's state-sanctioned media has suddenly made it a topic of serious discussion.

Such a step would be tantamount to "a nuclear bomb" lobbed toward Pyongyang, says Cheng Xiaohe, deputy director of the Centre for China's International Strategic Studies at Renmin University. "That would probably be the last economic resort."

But Beijing has a menu of lesser options that it could consider first, including evicting the tens of thousands of North Korean labourers who have been allowed into China in recent years to work in factories and restaurants.

Donald Trump and Xi Jinping pause for photographs at Mr. Trump’s Mar-a-Lago estate in Florida on April 7, 2017. ALEX BRANDON/ASSOCIATED PRESS

With the U.S. saying it is sailing an aircraft-carrier group in the direction of North Korea, what China can accomplish is crucially important. Indeed, action from Beijing amounts to the first phase of the Trump administration's strategy on North Korea, as it seeks to force the country to give up its nuclear program, says Stephan Haggard, director of the Korea-Pacific Program at the University of California San Diego and co-author of North Korea: Witness to Transformation, a blog that tracks economic and political change in the country.

The White House view amounts to "Let's give the Chinese a chance," Prof. Haggard insists. Although U.S. Vice-President Mike Pence last week warned that "the era of strategic patience is over" on North Korea, that proclamation "is for domestic consumption," Prof. Haggard says. "It doesn't have anything to do with what the administration is actually doing, which is precisely to wait to see what the Chinese can or can't deliver."

Whatever its downsides, such an approach is no doubt more palatable to Beijing than are such alternatives as secondary sanctions that target Chinese companies doing business with North Korea – or, the worst-case scenario short of nuclear war, a U.S. military strike.

In fact, there are signs that China has already begun quietly experimenting with new measures of its own. Take, for instance, those North Korean workers who were invited into China in recent years, provided visas, and allowed to labour as a source of foreign currency for Pyongyang. "Some of the Chinese companies in northeast China have stopped hiring DPRK labour," says Prof. Cheng.

Though the steps are modest, Prof. Cheng said they amount to Beijing moving beyond merely supporting United Nations sanctions, which already ban the sale of arms and luxury-goods materials to North Korea and place strict limits on countries' purchasing North Korean minerals and moving its money. China is now "taking unilateral action against the DPRK, even though China has consistently rejected this idea in the past," Prof. Cheng says. "China is now punishing the DPRK, in order to pressure it to come to the negotiating table."

Those who trade on the northeastern border say that other changes have also quietly slipped into place, although it's not clear how widely they are being applied. About six months ago, the Chinese fuel that Cui Yanzhi once bought inside North Korea, where he operates a seafood-products factory in a special region open to foreign development, was suddenly replaced by Russian-supplied energy.

Foreign journalists stationed in North Korea have recently documented long lines, and rising prices, at gas stations – although the reasons are not clear, and Chinese authorities have not given a clear answer about whether Beijing has cut supplies.

China has, however, taken seriously its February pledge to halt, for the duration of 2017, imports of North Korean coal, whose sale earns an estimated 40 per cent of the regime's foreign currency. On the Chinese coast, ships with North Korean coal have been turned back. (Some have recently been allowed to dock in China, although it's not clear that they were allowed to unload cargo.)

In an interview, Mr. Cui said that one of his friends bought 10,000 tonnes of coal and loaded it onto a ship that sailed out of Nampo, a port southwest of Pyongyang. But China barred the ship from its ports. "My friend is almost bankrupt," Mr. Cui says. He sent a message asking if the friend would speak with a journalist, and received a plaintive response: "The moment he begins importing my coal," said the friend, "I will talk."

Chinese imports of North Korean gold, iron, tin and rare earths all stopped last year, he said, further evidence of China's adhering to sanctions. "Big bulk-goods trade of any kind," Mr. Cui says, "is all now impossible."

Korean-, Chinese- and Russian-language signs hang above the stores in Hunchun, a Chinese city sandwiched between North Korea and Russia. NATHAN VANDERKLIPPE/THE GLOBE AND MAIL

Trade with North Korea continues in Hunchun, but there are also signs of a strict new approach from Beijing. NATHAN VANDERKLIPPE/THE GLOBE AND MAIL

On the ground, insouciance

The goods that continue to cross the border into China include edible wild plants, honey – and the seafood that Mr. Cui imports in large quantities.

For him, little has changed.

His store is situated in Hunchun, a Chinese city of 226,000 located inside a finger of the country that sticks south into North Korea alongside the Tumen River. Here, street signs, as well as those on establishments, are in Chinese, Korean and Russian, a visual reminder of the quirk of geography that has brought the three countries together in a single corner of land. The Russian border is 10 kilometres from Hunchun; Vladivostok is just 135 km away.

Mr. Cui runs a wholesale showroom on a street lined with seafood sellers and with karaoke bars whose front windows are populated by clusters of women waiting for male customers. But the company's real work takes place a short drive away, inside the Rason Special Economic Zone, a corner of far northeastern territory that Pyongyang has opened to Chinese investment, its roads built by China, its signs posted in Chinese, English and Korean.

There, Mr. Cui employs 100 North Koreans at his factory, which packages locally caught crab, shrimp and other seafood to be sold across China, using local workers who can be hired for as little as $60 a month, a small fraction of Chinese labour costs.

The prospect of military conflict doesn't much worry Mr. Cui. If fighting breaks out, "we will make a fortune," he predicts, confident that Chinese troops would move into North Korea in order to keep refugees on home soil, thus allowing his business to continue unaffected.

Such insouciance is commonplace in an area where decades of rising and falling tensions have bred boredom – and disdain for those who suggest that this time is any different from previous rounds of anxiety. In Yanji, Chinese tourist agencies have continued booking trips across the border to nearby towns and tourist attractions.

At the Chinese border town of Tumen, tourists still gather on a local boardwalk to gaze across at portraits of Kim Il-sung and Kim Jong-il, and at the North Korean guards patrolling the far banks of the river on foot. They feel little cause for concern. "The U.S. is just posturing in order to frighten North Korea," says Wang Jianbin, a young traveller from China's Hebei province, who travelled here with a friend.

A short drive away, near the city of Hoeryong, North Korean border guards engage in an intense game of outdoor volleyball, their shouts echoing up to a small Chinese hill overlooking the scene. The game disbands when trucks arrive at the two-lane concrete bridge that forms the border here, a picture of peaceful order.

Then, moments later, an SUV filled with Chinese men in uniform arrives at the overlook. They are bearing binoculars, a reminder that this is the front line of a place that North Korea itself has termed "the world's biggest hot spot."

Still, even with recent tensions, "it's no problem getting across the border. It's the same as usual," says Han Bing, another Hunchun businessman with a seafood-packaging factory in North Korea. There is a disconnect, he said during an interview in his store – which was jammed with North Korean shellfish and Russian chocolate – between fear on television and what he sees on the ground. Prices are the same. Business continues.

Besides, he says, the idea of violent disruption in a place pinched between the world's powers – Russia, China and the U.S. – seems so cataclysmic that it's not even worth contemplating. "Wouldn't that mean the start to World War Three?" he asks. That would "not be the same as the Second World War. It would be a nuclear war. And once it starts, the Earth will be destroyed."

A North Korean navy truck carries the submarine-launched ballistic missile during a military parade marking the 105th birth anniversary of the country’s founding father, Kim Il-sung, in Pyongyang on April 15, 2017. DAMIR SAGOLJ/REUTERS

With reporting by Yu Mei

Nathan VanderKlippe is The Globe and Mail's correspondent in Beijing.

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