北京議定書
Under the terms of the Boxer Protocol of 7 September 1901, China had granted nations with legations in Beijing the right to station guards at twelve specific points along railways connecting Beijing with Tianjin. This was to ensure open communications between the capital and the port. By a supplementary agreement on 15 July 1902, these forces were allowed to conduct maneuvers without informing the authorities of other nations in China.By July 1937, Japan had expanded its forces in China to an estimated 7,000 to 15,000 men, mostly along the railways. This number of men, and the amount of concomitant matériel, was several times the size of the detachments deployed by the European powers, and greatly in excess of the limits set by the Boxer Protocol. By this time, the Imperial Japanese Army had already surrounded Beijing and Tianjin.