Since the Japanese invasion of Manchuria in 1931, there had been many small incidents along the rail line connecting Beijing with the port of Tianjin, but all had subsided. In 1937, the Marco Polo Bridge Incident occurred. The Japanese, who were missing a soldier after a training exercise, demanded entry into a Chinese garrison to search for him. When the Chinese refused this, the Japanese attempted to force their way in, and tensions escalated until conflict broke out.

The Japanese government profited by a quite unimportant incident, extending its dominion a little further. Japan brought up reinforcements and put forward demands which China could not be expected to be ready to fulfil. Japan then occupied Beijing and Tianjin and wide regions between them and south of them. The Chinese soldiers stationed there withdrew almost without striking a blow, but formed up again and began to offer resistance. In order to facilitate the planned occupation of North China, including the province of Shandong, Japan decided on a diversionary campaign against Shanghai.

The Nanjing government sent its best troops to the new front, and held it for nearly three months against superior forces; but meanwhile the Japanese steadily advanced in North China. On November 9th Nanjing fell into their hands. By the beginning of January 1938, the province of Shandong had also been conquered.