As a matter of fact, as early as July of 12, the Japanese Navy Military Command made a secret plan to fight against China, and decided to cooperate with the army in North China in the first stage, attack Shanghai in the second stage with the cooperation of the army, and then expand the war to Central China and South China. On July 16, Ito Tanigawa, commander of the Third Fleet of the Japanese Navy, immediately wrote a letter to Tokyo, stating "opinions on fighting against China".
On July 28th, the Japanese authorities ordered the evacuation of overseas Chinese along the Yangtze River. On August 8, the Third Fleet was asked to prepare for the war.
On August 9 18: 30, Captain Daiyama Yong and sailor Saito of the Japanese Marine Corps in Shanghai tried to hide and drove into the cordon of Shanghai Hongqiao Airport. China security team failed to stop them and killed them on the spot, which is called "Hongqiao Incident" in history.
The Japanese authorities immediately took advantage of this incident to make a fuss, demanding that the China government withdraw from the Shanghai security team and dismantle all fortifications. On August 10, the Japanese Lu Haijun negotiated urgently and sent troops to Shanghai, which was approved by the Cabinet.
13 at 9: 00 on August, the Japanese cabinet meeting formally determined the policy of sending troops to Shanghai and approved the case of sending troops to the army province. On the morning of the same day, the Japanese Marine Corps had an armed conflict with the China Garrison between Shanghai North Station and Sichuan North Road. Relying on Hongkou Japanese Concession, the Japanese army first attacked Tiantong 'an and Hengbang Road, and then attacked Baoshan Road, Baziqiao and Tiantong 'an with the main force, all of which were defeated by China army. Thus, the "August 13th" Shanghai Incident broke out.