The day had dawned clear and sunny on Aug. 6, 1945. Sunao Tsuboi, an engineering student at Hiroshima University, was hurrying to class after quickly downing a bowl of porridge and slurping some seaweed soup at a roadside breakfast shack.

Okinawa had fallen to American troops, but Mr. Tsuboi doubted that Japan’s defeat was imminent. “I firmly believed the emperor was God, and I was ready to die for him,” he says.

Suddenly the young man was swept off his feet and hurled 30 feet by a deafening blast – the fury from the first use of an atomic bomb in history.

“Before I hid my face in my hands I saw a brilliant rosy-silver flash of light,” he recalls. He was briefly knocked out. “Then I found myself lying on the sidewalk, burned from head to toe. I couldn’t see anything except for smoke and dust.”

Tsuboi staggered around in a daze. Nobody helped him, and he finally collapsed. Lying prostrate, he found a small piece of rubble and used it to scrawl a last message in the dust that coated the ground so thickly: “Tsuboi died here.”

He didn’t die. A group of soldiers picked him up and took him to a hospital. But to this day Tsuboi bears the marks of his ordeal – a scarred forehead and a mutilated ear.

He still does not know how he survived. But his near-death experience led Tsuboi, who just celebrated his 90th birthday, to spend much of the past 70 years “reflecting on how humans might make peace,” he says.

The memorial to those killed in Hiroshima is an essential reminder. But “Japan has always talked about its own suffering” and emphasized its victimhood, Tsuboi says. “There should be more memorials to those who suffered from Japanese aggression.”

Seventy years after the end of the war in the Pacific, Japan’s neighbors say they are still waiting for an apology they can believe is a sincere expression of Japan’s national feeling about the old Imperial Army’s invasions and the atrocities it committed. Japanese Prime Minister Shinzo Abe is showing no signs he is ready to express the type of contrition that some of his predecessors have voiced in the past. The ramifications of this are now reverberating around the globe.

What might seem to much of the world like arcane disputes over incidents in 20th-century history are threatening Asia’s future and complicating the big power rivalry between the United States and China. They could strengthen Beijing’s standing in the region at the expense of Washington’s clout. They have poisoned relations between the publics and governments of America’s two closest Asian allies, Japan and South Korea, to the point where their leaders cannot meet.

And as Tokyo – emerging from a seven-decade cocoon of pacifism – strives to play a greater military role to support the US, its ambitions are spawning new concerns. Japan’s reluctance to face up to its past, former victims worry, means the country might return to its militarist ways, aggravating tensions with its neighbors.

“History is the core concern of Northeast Asian politics,” says Park Joon-woo, recently retired adviser to South Korean President Park Geun-hye. “And that will continue as long as Japanese leaders try to erase their history.”

“There is a lot of combustible material around, and Abe could make things pretty hot,” warns Thomas Berger, a professor at Boston University who has written a book about Japan’s postwar politics.

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At the forefront of those demanding that Japan do more to atone for its wartime legacy are China and South Korea.

The Chinese still bristle at the Japanese occupation of their country’s eastern reaches. They are especially bitter over the “Rape of Nanking” in 1937-38, when Japanese troops captured the city now known as Nanjing, which was then the Chinese capital. Some 200,000 people were killed and tens of thousands of women raped by marauding soldiers, according to the judgment of an international war crimes court.

South Koreans still harbor painful memories from 35 years of Japanese colonial occupation. They have focused their resentment on the fate of “comfort women,” Korean women and girls forced to work as prostitutes in Japan’s wartime military brothels.

Past Japanese leaders have apologized dozens of times for their country’s behavior in terms varying from remorse and regret to sorrow and repentance. The clearest statement came 20 years ago, in 1995, when then-Prime Minister Tomiichi Murayama acknowledged Tokyo’s history of “colonial rule and aggression” and offered his “heartfelt apology” for “these irrefutable facts of history.”

At the time, Japan’s neighbors accepted the apologies. When Japanese Cabinet Secretary Yohei Kono acknowledged publicly in 1993 that the Japanese Army had been involved in the forced recruitment of Korean sex slaves, “we were sure that was an expression of state policy that subsequent governments would continue to honor,” says Han Sung-joo, South Korea’s foreign minister at the time.

Recently, however, Japanese leaders have planted seeds of doubt about Tokyo’s sincerity. Most provocatively, in December 2013, Mr. Abe marked the first anniversary of his election by visiting the Yasukuni shrine, where Japan’s war dead – including more than 1,000 convicted war criminals – are commemorated. He had earlier infuriated the Chinese government by arguing in a parliamentary debate that “the definition of aggression has yet to be established in academia or in the international community,” suggesting that Japan had not really invaded China.

Bolstering regional suspicions that he is not genuinely repentant, the prime minister has taken no action against big-city mayors and political allies and appointees who have in recent years denied that the Nanjing Massacre ever happened, or said that the Imperial Army’s sex slave system was “necessary” or normal. Abe also expressed anger that McGraw Hill had not amended a passage about comfort women in one of its history books, as Japanese diplomats had pressed the US publishing company to do.

“Abe finds ways of signaling to his domestic audience” what he thinks, says Professor Berger, author of “War, Guilt, and World Politics After World War II,” and “nobody has any doubts where he stands.”

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That stand, say people who know the prime minister, is based on a belief that to restore Japanese pride and unity of purpose – to which he would harness a more ambitious military role in the region – the country should restore a more positive view of history.

Nor has anyone in Japan forgotten that Abe is the grandson of Nobusuke Kishi, Japan’s wartime munitions minister who was detained for three years as a suspected “Class A” war criminal by the Allied powers but never indicted or tried. Later, as prime minister, Kishi sought to change the pacifist Constitution, much as Abe is doing now.

“Mr. Kishi thought we had fought a just war against America, and Abe is Kishi’s reincarnation,” says Hideaki Kase, head of the Tokyo branch of Nippon Kaigi, an influential revisionist lobbying group that venerates the imperial tradition and denies that Japanese troops committed any war crimes. Fourteen members of the current Japanese cabinet and a majority of parliament members are associated with the group.

The revisionist current runs strongly through Japan’s political class, many of whose members are still linked by family ties to prewar governments. It finds few adherents, though, among professional historians, or among the general public, where Abe’s popularity rests on his promises of economic recovery more than on his nationalism.

Rather, ordinary citizens suffer from a general ignorance of Japan’s colonial past that startles outsiders and worries some Japanese. Even Shinichi Kitaoka, the man heading the team of advisers Abe handpicked to help him draft a statement to mark the 70th anniversary of the end of the war, is concerned.

“Not only young people but also many other sections of Japanese society know too little about modern and contemporary history,” he lamented in a recent opinion piece published by the conservative daily Yomiuri Shimbun.

Such ignorance is encouraged by Japanese textbooks, published privately but subject to government approval. Of eight junior high school history textbooks approved this year, only one mentions comfort women. And the Education Ministry insisted on adding a caveat to that one, noting that “the current Japanese government has expressed its view that ‘no document directly proving so-called forcible recruitment by the military and authorities has been found.’ ”

That runs counter to the admission by Mr. Kono 22 years ago, points out Yoshifumi Tawara, who runs a textbook watchdog group. It also, he argues, betrays the cabinet secretary’s written pledge “never to repeat the same mistake by forever engraving such issues in our memories through the study and teaching of history.”

Certainly no alternative views exist in the entrance hall to the museum honoring Japan’s war dead at Yasukuni, where a grand old locomotive stands proudly. An explanatory plaque tells visitors this was the first engine to travel the length of the Burma Railway. It says nothing about the estimated 100,000 slave laborers, including 12,000 Allied POWs alongside Chinese, Thai, and Burmese civilians, who died building the track. Nor does it mention the 32 Japanese overseers sentenced to death for war crimes because of the brutality they inflicted on the workers.

• • •

These sorts of omissions make it harder for Japanese citizens to understand just why their South Korean and Chinese neighbors might resent them. A tide of emotion is rising on all sides over territorial disputes: Japan holds islands in the East China Sea that Beijing claims, and South Korea occupies an island that Tokyo claims belongs to Japan.

These sovereignty disputes are inflaming the historical divisions, and the results are dramatic. A recent poll found that 73 percent of South Koreans have a negative view of Japan, and nearly two-thirds of them feel that Japan is a bigger military threat to the region than China is.

In Japan, a government poll at the end of last year found that 83 percent of Japanese have no friendly feelings for China, and 66 percent do not feel friendly toward South Korea.

Such widespread negativity “is not healthy,” says Yu Myung-hwan, a former South Korean foreign minister who has led a delegation of diplomats to both Abe and Ms. Park to convince them to meet. “Before that gets rooted in people’s mind-sets we have to change the atmosphere.”

Neither the Chinese nor the Korean governments are currently doing much to ease tensions, though. For months, the official Chinese media have been full of exhortations for Japan to “show a correct understanding of history.” In Beijing, the ruling Communist Party is planning an unprecedented military parade this year for Sept. 3, the date that China celebrates Japan’s surrender, playing to growing nationalist sentiment.

Such feelings feed into Chinese President Xi Jinping’s campaign for the “rejuvenation” of China under the rule of the party that helped liberate the country from the Japanese. It builds social and political cohesion, says Daqing Yang, who teaches international affairs at The George Washington University, especially after 25 years of “patriotic education” programs that have emphasized past Chinese suffering at the hands of foreign invaders and fueled anti-Japanese sentiment.

The depth of the animosities are evident at the Nanjing Massacre Museum, a somber edifice in black granite that every Chinese tourist visits. At the entrance stands a statue of a woman fleeing, a babe in her arms. “Run, the devils are coming,” reads the title, referring to approaching Japanese soldiers. A similar sculpture is called “Run away from the devils’ bloodbath.”

In South Korea, Park has seized on the issue of comfort women, responding to strong public sympathy for the 50 women still alive who have talked about their past as prostitutes in Japanese “comfort stations.” Each Wednesday at noon two of the survivors lead a demonstration across from the Japanese embassy, demanding justice. A bronze statue of a young comfort woman, an empty chair by her side to symbolize those who have died, also sits in mute reproach outside the embassy’s entrance.

Japanese and Korean diplomats have met eight times in recent months to try to hammer out a deal that would likely include Japanese apologies, reparation payments, and an acceptance of responsibility. Park has ruled out a summit with Abe until the issue is resolved. But the group has failed to reach an agreement that would satisfy the surviving comfort women without leaving Tokyo vulnerable to an avalanche of lawsuits.

“It’s not that China and Korea won’t let go of these issues,” insists Zhang Xianwen, a Chinese historian. “It’s that Japan has not sincerely recognized the historical issues and keeps changing its stance.”

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That has something to do with what happened in Japan after the war. In Germany, the Nazi Party was dissolved and no prominent Nazi ever played a role in politics again. In Japan, the US occupiers left the imperial system in place and many leading wartime figures played major roles in government well into the 1960s.

“The conservatives got off the hook fairly easily, so they lost an opportunity to repent more seriously,” says Koichi Nakano, a professor of politics at Sophia University in Tokyo.

The rulers in neighboring countries accepted that. South Korea normalized diplomatic relations with Japan in 1965 in a treaty that made no mention of comfort women. China normalized relations in 1972, waiving any demand for reparations on the grounds that the Japanese people had been as much victims of the militarists as had the Chinese.

Mao Zedong airbrushed awkward facts out of history in the interest of neighborly relations. Zhang Sheng, now head of the history department at Nanjing University, says he never learned about the Nanjing Massacre at school, for example. “History issues have been fermenting for many years, and they have become the starting point for how people discuss the future,” Professor Zhang says.

As civil society blossomed in South Korea in the late 1980s in the wake of a long military dictatorship, comfort women came forward and won public sympathy. In China, after the 1989 Tiananmen Square protests, the government channeled popular political sentiment into nationalism. Japan came under international pressure to repent – pressure that has only grown over the past quarter century as its former enemies have become global economic powerhouses.

Ten years ago, on the 60th anniversary of Japan’s surrender, then-Prime Minister Junichiro Koizumi used the same words of apology that Mr. Murayama had used a decade earlier, referring specifically to Japan’s “colonial rule and aggression.”

Abe is giving signals that he will not follow suit on Aug. 15, perhaps emboldened by polls showing that 57 percent of Japanese think their governments have apologized enough. “He is clear that he wants to make his own statement,” says one of his advisers. “He wants each and every word to be new.”

Abe has said that he sees no need to repeat previous statements since he has declared that he upholds them. But “he is certainly wrong if he thinks that will satisfy his neighbors,” says Berger. “It is really important that he uses the language Murayama used; if he doesn’t it will be a big problem for East Asia, for the US, and for Japan.”

Tokyo appears to have given up hope of winning understanding from Beijing and Seoul. “Urging Japan to apologize has become a tool for South Korea and China to wield influence,” says Abe’s adviser. “We need to send a convincing message to Europe and to the US. Winning the trust of our Western democratic allies is even more important” than winning trust from neighbors.

The Japanese government seems somewhat comforted that, despite the hostile Chinese rhetoric, President Xi has met with Abe twice in recent months after freezing him out. And in Seoul, influential voices are calling for compromise on the issue of comfort women.

“The US and Japan are moving forward as allies, and we are being held back because we cannot get out of the past,” says Lee Jung-hoon, the South Korean government’s ambassador for human rights. “Historical aspects are overwhelming the security dimension, and people are wondering whether this is right.”

In recent weeks, there have been tentative signs of a thaw. At a muted celebration in Tokyo of a half century of diplomatic relations, for example, South Korean Foreign Minister Yun Byung-se read a message from Park about the need to cast off “the heavy burden of history.”

If Abe brushes off regional resentments in his August statement, though, that history is likely to reassert itself with renewed vigor. The doubts that Abe has sowed “undermine US security policy in East Asia” as Washington tries to size up China’s goals, says Daniel Sneider of Stanford University, who studies historical memory in East Asia.

If Abe does not repeat Murayama’s language, “it will make it almost impossible for a Korean government to engage in reconciliation and will give China a wonderful weapon with which to hit Japan over the head,” Mr. Sneider says. “Beijing will continue its efforts to drive a wedge between Washington and Tokyo, which is one of its key goals.”

Japanese voters, too, have doubts about Abe’s intentions as he seeks to extend Japan’s military reach and reinterpret the Constitution to allow that. “Abe’s revisionism fuels Chinese opposition and domestic worries about his motivation,” says Akihisa Nagashima, a former deputy Defense minister from the opposition Democratic Party of Japan. “We should separate history issues from the necessary effort to normalize our security situation.”

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Can japan and its neighbors ever overcome their history? Can they reach further back into their past to their common philosophical heritage and find the sort of reconciliation that Germany has built with its European partners?

Berger, who draws on the experiences of Germany and Austria to identify several conditions for reconciliation, is not hopeful in the short term. Japan’s leaders lack the political will, he thinks. China and South Korea are not yet ready to reciprocate by accepting apologies. There is no consistent message from Japan, where senior officials get away with provocatively revisionist claims.

“Damage control” is probably the best that can be expected for the time being, Berger says.

Still, mutual interests could soften attitudes. Sven Saaler, who teaches memory studies at Tokyo’s Sophia University, says businesspeople in Japan are keen to see relations with China improve. Japanese investment in China fell by nearly 40 percent last year from 2013. Samsung would prefer to put its name and logo on its flagship Galaxy 6 smartphone, which it now sells in Japan incognito, for fear of negative consumer reactions to a South Korean name.

China has a lot to learn from Japan in areas such as the environment and how to cope with aging populations, Dr. Yang points out, while Japan is delighted by the flood of Chinese tourists taking advantage of the weak yen. “There are areas of common interest,” says Yang. “They may not be decisive, but they make a positive difference.”

Collaborations are also under way on history books. While official projects have foundered on the rocks of political correctness, historians from China, Japan, and South Korea have unofficially found it possible to publish textbooks such as “A History to Open the Future,” offering middle-schoolers a single account of the region’s past from medieval times to the present.

Though not an official text in any of the three countries, teachers do use it as a study aid, says Bu Ping, a historian who led the Chinese team that helped write it. That is all very well, he adds, but “without positive efforts” by politicians, collaborative projects such as his have little effect.

Abe certainly has the patriotic credentials to offer reconciliation. “If he wants support in Asia and to be a world leader, he has no option,” says Kazuhiko Togo, a retired high-ranking Japanese diplomat. But Mr. Kase, the revisionist leader, is confident Abe will do no such thing.

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“My people are in power now,” he boasts. “And we must regain our independence ... and pride in our history.”

If Kase’s confidence in Abe is not misplaced, reconciliation between Japan and its neighbors will have to wait until a new generation of political leaders takes power in Tokyo. And then, perhaps, predicts Professor Nakano, “if Japan is seen as sincere, it won’t be blamed forever for everything.”