The Marco Polo Bridge Incident was a July 1937 battle between China's National Revolutionary Army and the Imperial Japanese Army. 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. On this occasion, a Japanese soldier was temporarily absent from his unit opposite Wanping, and the Japanese commander demanded the right to search the town for him. When this was refused, other units on both sides were put on alert; with tension rising, the Chinese Army fired on the Japanese Army, which further escalated the situation, even though the missing Japanese soldier had returned to his lines. The Marco Polo Bridge Incident is generally regarded as the start of the Second Sino-Japanese War, and arguably World War II.