Guo Jingxing and his wife Jiang Yaxian were primary school pupils when shots rang out near the Marco Polo Bridge late on July 7, 1937. They are marking the 70th anniversary of the incident by publishing the first book on the mainland to focus on what happened before, during and after that night. The seeds of the book, In Memory of the July 7 Incident, were sown in 1981 when Mr Guo was unexpectedly assigned to head the bridge's Heritage Protection Office. He often found himself out of his depth when visitors asked simple questions, such as where the battle was fought and how negotiations were conducted with the Japanese. 'There is no detailed record of this segment of history,' Ms Jiang said. Mr Guo said one reason for the lack of detailed information was that people who reported on the battle were not the ones who had fought. Also, the country's military system at that time made no proper records or notices of battles fought. Reporters did not cover the Marco Polo Bridge Incident because the Beijing authorities were struggling to maintain a fragile co-existence between Chinese and Japanese troops. 'This is not only the battlefield that marks the beginning of China's anti-Japanese war, but also the first battlefield of the second world war,' Ms Jiang said. The battle took place two years before Germany invaded Poland in 1939. 'If we do not get to the bottom of this war, we will have failed in our duty to our descendants,' she said. Mr Guo and Ms Jiang are not historians - both are former high school language teachers - but they are motivated by a sense of mission to record the little-known history of the incident before it is too late. That mission has taken them on many trips to Henan , Nanjing , Shanghai and Taiyuan in Shanxi , to seek first-hand accounts from officials involved in the battle and residents who witnessed it. For more than 20 years, the couple tried to track down survivors and the descendants of those who had died, urging them to release relevant historical documents and artefacts. They also organised many forums for witnesses to meet and tell their stories. The 800-year-old bridge in southwestern Beijing, celebrated for its beautiful form and carved stone lions, runs east to a walled community called Wanping, which was an important bastion for the defence of the southwestern gateway to the capital. It was also the meeting point of three strategic railways. Among the key people the couple interviewed were Jin Zhenzhong, chief of the 29th Army battalion that fought the battle, Wanping government secretary Hong Dazhong and some other officers caught in the battle. In addition, they tracked down the sons of two other main players in the incident - Wanping county chief Wang Lengzhai and Ji Xingwen, an officer leading a brigade responsible for safeguarding Wanping and the bridge. Their sons were in the United States and Taiwan respectively. The couple also relied on many letters, diaries, history books and battle notices collected by Japanese friends. Mr Guo said there were no more survivors of the Marco Polo Bridge conflict, and most parts of this and other battlefields had changed beyond recognition. Since taking up the heritage protection job, Mr Guo has initiated the restoration of the bridge. He has also promoted the conservation of Wanping and founded the Anti-Japanese War Museum, built within its walls in 1987. 'It is very important to let people know the true details of the Japanese invasion and reach a conclusion on who should bear the responsibility for the war,' Mr Guo said. 'We hope these details can help us find justice.'