Marco Polo Bridge incident

The Marco Polo Bridge incident, also known as the Lugou Bridge incident or the July 7 incident, was a three-day battle that began on 7 July 1937 in the district of Beijing between the 29th Army of the National Revolutionary Army of the Republic of China 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.

Source: Wikipedia — Marco Polo Bridge incident (CC BY-SA 4.0)