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The tour guide outside the bloodstained classrooms of the Tuol Sleng Genocide Museum, the high school in the Cambodian capital of Phnom Penh transformed into a prison and torture center by the Khmer Rouge, paused to ask whether any tourists in the group were from China. Visibly relieved when no hands were raised, he went on to describe the enabling role that Beijing played in the Khmer Rouge’s murderous rampage that claimed the lives of an estimated 1.7 million Cambodians beginning in 1975.

Later, he explained why he asked whether there were Chinese among his audience. “They get very angry when I say it was because of China that Pol Pot was able to kill so many people,” he said with evident frustration. “They claim it’s not true, and then say ‘We are friends now. Do not talk about the past.’”

As China prepares to commemorate the 70th anniversary of the end of World War II with a military parade in Beijing in September, the state news media has been hammering away at a central theme underpinning the government’s narrative about the suffering China endured under Japanese occupation: Tokyo must “face history,” goes the storyline and reaffirm its admitted wrongdoings. But China’s insistence that Japan face history is raising uncomfortable questions about Beijing’s own practice of suppressing historical truths about trespasses domestic and abroad.

Last week, People’s Daily, the Communist Party mouthpiece, published a series of articles that accused the Japanese government of “whitewashing its wartime past” and warning that right-wing nationalists were plotting to return the country to its militaristic ways, potentially jeopardizing regional stability.

Premier Li Keqiang of China raised the issue during a televised news conference this month. “For leaders of a country, while inheriting the historical achievements made by their predecessors, they also need to shoulder the historical responsibilities for crimes committed by past generations,” he said.

Pivoting off such statements, a number of independent Chinese historians have tried to highlight the Communist Party’s role in the deaths of tens of millions during man-made famines and the political terror that marked its first decades in power — episodes that are erased from the nation’s official history.

“The Chinese government propagandizes the parts which it finds useful while ignoring aspects that could draw criticism,” said Zhang Lifan, a prominent historian who has sought to illuminate the party’s selective approach to its history, which is enforced through media censorship and book-publishing bans.

In recent months, the Japanese, too, have been calling on China to acknowledge its role in some of the greatest atrocities of the 20th century.

Writing for the Japanese website JBpress, Kuni Miyake, a retired Japanese diplomat, castigated the Chinese government for mocking “the global standard of intellectual fairness” by refusing to accept accountability for the Great Leap Forward, Mao Zedong’s failed industrialization effort during the 1950s that some historians say led to the death of 45 million people by famine and other causes, as well as the 1966-76 Cultural Revolution that killed thousands and traumatized a generation.

“If China asks others not to whitewash the history of 80 years ago, Beijing should be able to also face the modern history of China in the 1950s, ’60s, ’70s and, of course, in 1989,” Mr. Miyake wrote, the last date a reference to the year Chinese troops gunned down unarmed civilians during the protests at Tiananmen Square. “So far, there are no history museums in China that face such history.”

In recent months, Beijing has repeatedly expressed consternation with Prime Minister Shinzo Abe of Japan, a conservative who has sought to play down his country’s wartime atrocities in Asia while denying that thousands of “comfort” women and girls were forced into sexual slavery for Japanese soldiers.

Yet the Chinese government has been just as adamant in rejecting any parallels between Tokyo’s revisionist tendencies and its own refusal to acknowledge the tragedies that scar the nation’s recent past. “They are like wind, horse and cow, completely unrelated,” the Chinese Ministry of Foreign Affairs wrote in response to faxed questions.

But in Cambodia, a small band of historians has been clamoring for Beijing to acknowledge its role in one of the worst genocides in recent history.

In the 1970s, Mao wanted a client state in the developing world to match the Cold War influence of the United States and the Soviet Union. He found it in neighboring Cambodia. “To regard itself as rising power, China needed that type of accessory,” Andrew Mertha, author of “Brothers in Arms: China’s Aid to the Khmer Rouge, 1975-1979,” said in an interview.

According to Mr. Mertha, director of the China and Asia-Pacific Studies program at Cornell University, China provided at least 90 percent of the foreign aid given to the Khmer Rouge, from food and construction equipment to tanks, planes and artillery. Even as the government was massacring its own people, Chinese engineers and military advisers continued to train their Communist ally.

“Without China’s assistance, the Khmer Rouge regime would not have lasted a week,” he said.

In 2010, the Chinese ambassador to Cambodia, Zhang Jinfeng, offered a rare official acknowledgment of China’s support of the Khmer Rouge, but said that Beijing donated only “food, hoes and scythes.”

Citing records and testimony from former Khmer Rouge officials, Youk Chhang, a survivor of the genocide and executive director of the Documentation Center of Cambodia, disagreed. “Chinese advisers were there with the prison guards and all the way to the top leader,” Mr. Youk said. “China has never admitted or apologized for this.”

The Chinese government’s effort to shape the narrative about the nation’s past begins in schools. Four of the most widely used high-school history textbooks avoid any mention of the Khmer Rouge. They also omit China’s 1979 invasion of Vietnam, a monthlong war launched by Deng Xiaoping to punish the Vietnamese for toppling Pol Pot’s regime.

Unlike China’s battles against the Japanese, which often dominate prime-time television slots, the invasion of Vietnam gets scant screen time. The effort has been so successful that many university students in China have no idea that the war even took place.

The enforced historical amnesia about China’s invasion of Vietnam has come at a price. For years, thousands of the war’s veterans have complained of being denied benefits and adequate compensation for their role in the conflict. Many have been detained for protesting.

“I don’t think the government values us enough,” said Li Zizhong, 60, a veteran from the coastal city of Qingdao who has been petitioning the government for six years to increase his 350 renminbi (about $57) monthly subsidy. “Apart from that I have nothing.”

By contrast, Chinese textbooks go into great detail about the Korean War, officially known in China as “The War to Resist America and Aid Korea.” But Chinese textbooks ignore one pivotal detail of that conflict: that it started when North Korea invaded the South in June 1950. Instead, they state only that war “broke out.”

According to “War and Peace in the Twentieth Century,” a textbook published by the Chinese Ministry of Education, after United States troops “lit up the flames of war,” China was forced to secure the country’s “national safety and support the just cause of North Koreans which greatly enhanced the international status of China.”

Mr. Zhang, the historian, says the Communist Party’s refusal to permit an honest historical reckoning ultimately undermines China’s global standing.

“If China acknowledged its past one day and stopped hiding from history,” he said, “it would help on the world stage and win the party a lot more support from the Chinese people.”