'I read a lot of historical material to prepare for the film,' says Lu. 'A wealthy friend from Sichuan spent 200 million yuan [HK$227 million] on vast amounts of [Japanese] artefacts from the second world war - the letters Japanese soldiers wrote home, their diaries, wartime magazines and photographs. I was reading through those diaries and I was shocked to discover they were all human; they wrote about human matters and they appeared to have been very sober, ordinary people: they were definitely not madmen.'

In the film, Japanese soldiers are seen engaging in everyday activities: joking with each other by the river, having their hair cut by their comrades and playing football. The sight of these young men enjoying themselves and behaving so normally serves to bring their murderous deeds into sharper focus, Lu says.

'It is even more terrifying to see normal people go out and murder,' he says. 'It brings another layer of meaning to the film. We are all normal people, but would we take part in a massacre if we were caught up in a war? My way of seeing it is that [the Nanking Massacre] was not down to some problem with the Japanese cultural psyche; the problem lies with the notion of war itself. War makes a person, or an entire nation, crazy, and people do things they wouldn't normally have imagined doing.'

Even Kadokawa, who seems dazed and disgusted by the carnage his comrades have inflicted, plays a part in the violence: he's seen beheading Chinese soldiers who have surrendered.

Examining the Nanking Massacre from the point of view of a Japanese soldier is an approach similar to the one Clint Eastwood takes in Letters from Iwo Jima, which looks at the deadly battle that took place on the Pacific island at the end of the second world war through the eyes of Japanese soldiers.

'I hope this film helps convince people of the tragic nature of war; I don't want it to become a propaganda piece,' he says. 'It's more objective to see events from the point of view of the aggressor and I could only reveal the reasons behind such carnage by doing that. After all, it's [the Japanese soldiers] who killed, not ours.'

However, many people failed to appreciate Lu's raison d'?tre. The film might have generated 70 million yuan in its first week in the mainland last month, but it also attracted criticism for being too sympathetic to the Japanese.

'A lot of ultra-nationalists are saying I am not a Chinese director, questioning how I could view the Nanking Massacre from a Japanese perspective,' he says. 'A lot of people admonished me for making the film - some sent me e-mails saying they wanted to kill me.'

His detractors could find a lot of things to dislike about City of Life and Death. While the film begins with Japanese units bombarding Nanking, the first on-screen scuffle involves Chinese soldiers fighting each other, as mutinous units confront those whose units refuse to let them leave the city. Lu also depicts quislings among the Chinese civilians. One such figure is Mr Tang (played by Fan Wei, well-known to mainland audiences as a chubby comedian), the timid assistant of John Rabe, the German businessman who presides over a safety zone seeking to protect Chinese refugees from Japanese atrocities. He's seen befriending marauding Japanese soldiers and later sacrifices the lives of others in the zone in return for his own safety.

Mainland censors pored over the film's content for nearly five months before approving it for general release. Lu wasn't told why they took so long, but believes it might have been because they were concerned about its impact on Chinese-Japanese relations. '[The censors] might have passed the film around various people in the government to see if it would rouse ordinary folk to hit the streets and stone the Japanese ... but they can see it's okay. There haven't been any problems, I haven't heard of anyone burning down Japanese stores.'

The film hasn't yet secured a distribution deal in Japan, where far-right politicians and historians have for years denied that the Nanking Massacre ever occurred.

'The Japanese actors were in a very difficult position,' Lu says. 'They had to endure more pressure than they could have imagined. At first they thought it was just another film; after reading the script they were really taken aback. We actually recruited 90 Japanese actors; in the end, half of them didn't show up.'

He says the role was especially demanding on Hideo Nakaizumi, who plays Kadokawa, and Ryu Kohata, who plays Ida, the jaded career soldier who has grown accustomed to the brutality in his ranks. 'The younger Japanese actors couldn't imagine something like that happened and were shocked when I asked them to do scenes in which they rape and murder people,' says Lu. 'Ryu told me he couldn't sleep after shooting.'

Lu also hardly slept through the film's production. Financed by Hong Kong's Media Asia, China Film Group and two other mainland companies, the HK$80 million movie was only the third feature film of the director's career. Born in Xinjiang to Shanghainese parents - his father, Lu Tianming, is a famous writer - he studied in Nanjing before entering the Beijing Film Academy, graduating in 1998 with a master's degree in directing. His directorial debut, The Missing Gun, was lauded at the Venice Film Festival, but it was 2004's Kekexili, about a mountain patrol battling murderous antelope poachers in a Qinghai natural reserve, that propelled him to international fame.

City of Life and Death began in 2004 when a group of American producers approached Lu with a screenplay that combined The Rape of Nanking: The Forgotten Holocaust of World War II, Iris Chang's award-winning book about the Nanking Massacre, with The Good Man of Nanking, John Rabe's published diaries.

'We worked on it for a month or two but I didn't like the script - it's a story about Germans saving the Chinese and I don't think I could better Schindler's List in any way,' he says. 'This part of history is about the Chinese and Japanese - not Germans. It's more important for such a film to look at war and how it affects human beings.'

Inspired by the commission, Lu began writing his own script, which won an award at the Hong Kong-Asia Film Financing Forum in 2005.

Having met audiences on a seven-city tour around the country, Lu says he's satisfied with what he has achieved. 'Most of the youngsters loved the film - they probably haven't been instilled with that much poisonous dogma,' he says, laughing.

'They know this film aims to deliver a message of love and peace. A student told me he's seen the film four times - I do wonder why anyone would watch a film that many times, but I'm pleased the message got across.'

City of Life and Death opens today