The Chinese have rediscovered “their” second world war. Just as the conflict fades from memory in the west, it has become salient as never before in China. To understand the country today, we need to understand its long-forgotten war, argues Rana Mitter, professor of Chinese history and politics at Oxford. Remarkably, his new book is the first full account of the Sino-Japanese war ever published in English.

Perhaps 15 million Chinese died in the conflict, nearly 20 times the number of American and British war dead combined. Yet for decades China's ruling Communists rarely mentioned the war. After all, it hadn't particularly been their war. The Nationalist leader Chiang Kai-shek was China's main commander. The Communist party acted as his “junior partner”, writes Mitter. Indeed, in Mitter's account Mao Zedong is a relatively minor character, sitting out the war in the backwater of Yan'an. Occasionally Communist soldiers fought the Japanese, but during the war they also intermittently fought the Nationalists.

After Mao won the Chinese civil war in 1949, driving Chiang to Taiwan, he wasn't keen to talk up the feats of his defeated Nationalist enemy. That's why the long Japanese bombing of Nationalist-run Chongqing - China's equivalent of the London Blitz - was quietly remembered in Mao's day only by people who had lived through it.

Westerners all but forgot China's war. Under Mao, China became a closed communist country, whereas Japan was a western ally. Chinese archives were closed. Few western scholars could read Chinese anyway. And so China became, in Mitter's phrase, “the forgotten Allied power”. Westerners similarly undervalued the Soviet war effort until Russian archives opened in the 1990s.

Only in the 1980s did China start to commemorate the Sino-Japanese war as more than just a heroic Mao-led prelude to communist nirvana. In 1985 a museum opened in memory of the Japanese “Rape of Nanjing” of 1937-38 - a slaughter of up to 300,000 people that had never previously much interested the party because it hadn't been there. Today Nanjing is much-discussed in China; rather more so than bigger massacres of Chinese by Chinese, notably Mao's Great Leap Forward and the Cultural Revolution.

The post-Mao party revived the Sino-Japanese war, mostly because it needed a new ideology to replace communism. That need became urgent after the Tiananmen Square uprising of 1989. In a dictatorship, Mitter told me, “There are two things you can do. One is to run the economy really well, and the other is to make people feel nationalistically proud. Nationalism is a very powerful button to press.” Similarly in Yugoslavia, Slobodan Milosevic dropped communism for nationalism.

War talk had other uses for the new China. By talking up the shared battle of all Chinese, Communist and Nationalist, the party hoped to woo the Taiwanese. This hasn't always worked. As Mitter says: “Chinese praise for Chiang has coincided with a severe downgrading of shares in Chiang Kai-shek in Taiwan, where he's now regarded as a dictator who oppressed the people of Taiwan for many years.” War talk also sends a reminder to former western allies: that China, the new “responsible great power”, was with them when it mattered.

But in western countries, the second world war is fading into history. The Iraq war was probably the last time ever that an American president (with a bust of Winston Churchill in his office) would invoke shared Allied memories to cajole European countries into joint war. The European Union, built to unite a continent destroyed by war, has lost its sense of mission partly because the war is being forgotten.

In China, by contrast, the Sino-Japanese war now looms so large that it sometimes eludes the Communist party's control. Many Chinese citizens show an anti-Japanese fervour that embarrasses the leadership. For instance, Chinese officials generally find Japan's nostalgic rightwing prime minister Shinzo Abe (grandson of a suspected war criminal) a man they can do business with. But many ordinary Chinese get angry when, say, Abe makes tactless comments about the foreign “comfort women” used as sex slaves by Japan's wartime army.

Microbloggers on Weibo, the “Chinese Twitter”, often refer to the Japanese as “dwarf bandits” - a wartime (and Ming-era) insult. Sometimes popular anger goes beyond words. Last September, violent anti-Japanese demonstrations were broken up by Chinese police with water cannons. Popular anger may also be pushing China's new President Xi Jinping to hang tough in his dangerous dispute with Japan over the tiny islands in the East China Sea. Now some Chinese scholars and military officers are even claiming Okinawa for China. The Communist party is riding belligerent nationalism, but it is struggling to stay on the horse.