Oxford historian draws on new material to talk about one of the great untold stories of 20th century

Rana Mitter is determined to shed light on what is often seen in the West - although clearly not in China - as the forgotten war.

Despite killing up to 20 million people, including many savagely such as in the infamous Nanjing Massacre, and creating between 80 and 100 million refugees, China's War of Resistance against Japanese Aggression is often treated as a World War II sideshow.

"I thought it was one of the great untold stories of the 20th century and certainly the World War II period," he says.

"You can go into any history section in a bookshop and see endless books on Nazi Germany, the Blitz or the Americans in the Pacific war but it seemed to be that this story of a huge number of deaths, refugees and the destruction of the country that resulted in a huge change in the historical trajectory was often almost completely ignored."

With China's War With Japan 1937-1945: The Struggle for Survival, which is the result of 10 years of extensive research and is seen as one of this year's most important books on China, the Oxford historian aims to redress that balance.

Mitter, who was speaking in the front room of his terraced home in Oxford on a rainy spring day after an overnight flight from Washington, says the present day low profile of the war in the West is in contrast to how it was seen at the time.

"People were concerned with the spread of world fascism and it attracted almost as much attention as the Spanish Civil War. English writers like Christopher Isherwood and W.H. Auden traveled to China to report on it. It was not seen as a local thing then," he says.

Mitter's book is far from some Western history of the war. He was awarded a $1.3 million (992,000 euros) award from the Leverhulme Trust, a major academic funding body in the UK, in 2004 and that has enabled him to fund a team of researchers around the world.

Extensive use has been of material in the Chongqing Municipal Archive (from where China was governed for most of the war), the No 2 National Archive in Nanjing as well as the National Archives at the University of Maryland, College Park, which has major sources on America's China wartime experience.

The book also made use of Chinese nationalist leader Chiang Kai-shek's diary at the Hoover Institution at Stanford University, which had been made available to scholars.

"I was also able to draw on the fantastic scholarship on the Chinese mainland, Taiwan and Japan over the past 20 years," he says.

Mitter, 43, who is director-designate of the new Oxford University China Centre as well as a well-known BBC radio arts presenter, says it was a "multinational collaborative effort".

"I had a team of about eight or nine post-doctoral fellows and graduate students at Oxford who used to meet almost weekly to discuss different aspects of World War II."

This is also not some weighty tome but a very readable history with numerous gripping first-hand accounts that comes in at only 385 pages, partly on the insistence of the US publisher Houghton, Mifflin Harcourt which publishes it under another title - Forgotten Ally: China's World War II, 1937-45.

"I wanted it to be manageable. Books of more than 500 pages tend to be respected rather than read. I wanted it to read as a story with characters," he says.

With any history of this conflict, one wants to know how it treats the Massacre of Nanjing, where thousands of women were raped and many innocent Chinese civilians gruesomely slaughtered in what many see as a uniquely evil incident.

Mitter, who deals with it in a single chapter, makes clear it was a tragic episode. "In the crucible of total war soldiers behave very badly... Nanjing needs to be seen in its own terms."

"I don't think it is useful to have a ladder of comparative atrocity."

Mitter, a fluent Chinese speaker, says a risk for historians is getting diverted by the blood and gore of this conflict.

"I had had too many Japanese atrocities in the original version. The editors told me that to tell two of them in a detailed way was a better way of making your point than to have, say, seven," he says.

The book sets the war in historical context and how before the Japanese invasion of Manchuria in 1931, Japan was seen as a beacon of development in Asia and a place where a number of China's eventual war leaders, including Chiang himself, had studied.

"Japan can be seen as a monster and invader but had actually been a mentor for China. Unlike China, Japan had modernized and learnt fast that it needed to have a strong and developed economy," he says.

Although Japan was a more advanced country, it would be wrong to conclude the Chinese army was second rate. Some 30,000 of its officers had been trained by the German generals Hans von Seekt and Alexander von Falkenhausen and the resistance they provided helped deny Japan victory.

"There was a certain group of Chinese troops that were very well trained and this has been a discovery over the past 10 to 15 years by both Chinese and Western historians."

The game plan for China was to avoid defeat until they got international help, which came in the not so harmonious form of the Americans led by General "Vinegar Joe" Stilwell after Pearl Harbor.

"There is a very close link between the toxic relationship that grew between China and America and the failure to understand each other in the war years," he says.

Mitter says the war clearly still has legacies - China's permanent membership of the United Nations for one - and also on Sino-Japanese relations.

"There is something in the (Chinese) public culture that argues Japan has not properly atoned for the war and it remains a running sore, particularly in sensitive places like Chongqing and Nanjing. I think both sides still need to understand more about the history," he says.

China's War With Japan 1937-1945: The Struggle for Survival (Allen Lane) and Forgotten Ally: China's World War II, 1937-45 (Houghton, Mifflin Harcourt)