A group of disputed islands, Uotsuri island , Minamikojima and Kitakojima, known as Senkaku in Japan and Diaoyu in China is seen in the East China Sea

From the long-distance perspective of an American, Asia looks like one of the world’s most peaceful places. And it is — for the moment. But when Vice President Joe Biden arrived in Tokyo on Monday, he stepped into what has suddenly become a dangerous diplomatic crisis between China and Japan. On the surface, it’s a dull dispute over a string of uninhabited Pacific islands. Underneath, as I realized on a recent visit to China, it’s a story reaching back some 75 years that involves war, brutality, rape and historical reckoning. And now threatens to drag in the U.S.

The immediate cause of the crisis is Beijing’s recent declaration of an air-defense zone over the disputed islands, a string of rocks about 200 miles southeast of China’s coast, not far from Taiwan. No one will ever vacation on the islands, but there’s a good incentive to claim them, given that they sit in an area of the Pacific that may contain enough oil to fuel China for 45 years.

Far more than a story about energy, however, this is a story about national pride and historical grievance. The showdown over the islands — whose very name the two countries disagree about: China calls them the Diaoyu and Japan the Senkaku — touches one of the most sensitive nerves in Chinese culture: the Japanese occupation of China from 1937 to ’45.

“It may be hard for you to understand,” an expert at Beijing’s Academy of Military Science told me in October, echoing several others to whom I spoke. “The nationalist feeling, the emotion toward Japan, is very strong.”

It’s true that China has been flexing its muscles in recent years, by many accounts “bullying” regional neighbors like the Philippines and Vietnam. That’s one reason for the Obama Administration’s “pivot to Asia” strategy. But the rivalry between Beijing and Tokyo is much deeper, and darker.

Americans tend to think of World War II’s Pacific theater as a fight between the U.S. and Japan in places like Midway and Okinawa, lasting from 1942 to the atomic bombs of 1945. But for China, practically speaking, there was no “world” war — only a brutal invasion and occupation by Japan that began more than four years before Pearl Harbor. That fight cost 20 million Chinese lives, and included some of the worst atrocities of the entire war in the now infamous city of Nanjing, where up to 300,000 Chinese were massacred.

Few people with clear memories of those years are still alive today. But China has not forgotten. One Beijing-based American blogger calls the subject of Japan’s wartime conduct “radioactive … capable of eliciting such white-hot rage from seemingly normal people.” A few years ago, TIME’s then Beijing correspondent Matthew Forney captured how that rage is sustained:

Chinese kids can be forgiven for thinking Japan is a nation of “devils,” a slur used without embarrassment in polite Chinese society. They were raised to feel that way, and not just through cartoons. Starting in elementary school children learn reading, writing and the “Education in National Humiliation.” This last curriculum teaches that Japanese “bandits” brutalized China throughout the 1930s and would do so today given half a chance. Although European colonial powers receive their share of censure, the main goal is keeping memories of Japanese conquest fresh. Thousands of students each day, for instance, take class trips to the Anti-Japanese War Museum in Beijing to view grainy photos of war atrocities—women raped and disemboweled, corpses of children stacked like cordwood. As one 15-year-old girl in a blue and yellow school uniform, Ji Jilan, emerged from a recent visit to the gallery, she told a TIME correspondent: “After seeing this, I hate Japanese more than ever.”

Compounding the anger is a sense that Japan has never truly never reckoned with its atrocities the way Germany did after World War II. Japanese officials have issued statements of remorse over the years, but it’s typical to hear the Chinese say Japan has never really admitted to, or apologized for, its actions. China especially resents Japan’s lack of public contrition about the thousands of sex slaves known as “comfort women.”

Japanese officials inflame these wartime wounds every time they visit a notorious shrine to the country’s war dead that includes 14 wartime leaders convicted as war criminals by an allied tribunal; a government-linked Chinese newspaper recently described one such visit as an “attempt to deny and glorify the militarism and history of aggression.” Further stoking China’s paranoia is Japan’s recent move away from its postwar pacifist stance and toward a more potent military: “It is one of the greatest concerns for China if Japan tries to revise its constitution and have a normal military force,” said the military-academy expert.

Japan’s recent militarization is driven, in part, because the feeling is mutual: polling shows that the animus in Japan toward China runs about as high as it does on the other side of the East China Sea. The “unfavorable feelings” of each side toward the other runs poisonously above 90%. It’s certainly hard to argue that China has done anything to Japan comparable to the 1937–45 occupation. But one scholar on Sino-Japanese relations argues the animus is about envy and anxiety toward the roaring Chinese dragon:

… with its economic miracle stunted and political reform stagnating, many Japanese politicians played to nationalist groups in order to boost national confidence and win popular votes. While pacifism and the Japanese feeling of war guilt used to keep anti-Chinese nationalism marginal, the old culture has gradually faded, much due to people’s anxiety about an increasingly powerful and assertive China in East Asia.

How does all this affect the U.S.? In part because a conflict between Asia’s two great economic powers could be a disaster for the global economy; but also because of America’s security treaty with Japan, which, in return for basing rights, guarantees the U.S. will defend Japan against attack. And while Washington has not taken an official position on the sovereignty of the disputed islands, it does, somewhat confusingly, consider them covered by the security pact — a fact not appreciated by China, which is already wary about America’s intentions in the region.

“It just seems absurd that the U.S. would commit itself to the defense of a few small islands that it has no position about,” the military-academy expert told me. Most Americans would likely agree — if they even understood what the fuss was about to begin with.