Excerpts from Books and Wikipedia
"The battle that was responsible for sparking off the eight-year Sino-Japanese war and eventually led to the collapse of Japan's efforts to conquer China in the 1930s and 1940s took place in July 1937 around the medieval marble bridge over the Yongding River south of Beijing. . . . Japan had been steadily expanding its control of northern China from its puppet protectorate Manchukuo (Manchuria) since the Mukden incident of 1931. From 1932 to 1933 it rested control of Rehe province around the Great Wall and by mid-1937 controlled the northern, eastern, and western approaches to the ancient Chinese capital. . . . and at midnight the Japanese began bombarding the 1000 Chinese troops defending the bridge." [Furtado: 1001 Days] In July 1937, the Japanese invaded and occupied North and East China. The Nanjing Massacre occurred in December 1937. [Fall and Rise of China Guidebook, p. 178]