Short answers, pulled from the story.
The Marco Polo Bridge incident began on the 7th of July 1937 with a missing soldier search that escalated into full-scale fighting by 04:50 on the 8th of July. This event marked the initial battle of the Second Sino-Japanese War.
Chinese regimental commander Ji Xingwen led about 100 men to hold the bridge against superior Japanese artillery and infantry. General Song Zheyuan later ordered troops on heightened alert as precautionary measures before being defeated and forced to retreat behind the Yongding River by the 27th of July.
By July 1937, Japanese military strength in the region exceeded 15,000 men along railways connecting Beijing and Tianjin. These numbers greatly surpassed limits set by the Boxer Protocol of 1901 and grew to exceed 180,000 personnel by the 20th of July.
The eleven-arch granite bridge gained its Western name from Marco Polo's travel records in Il Milione. Local communities called it Lukouchiao or Lugouqiao, derived from the Yongding River's former name.
Historians debate whether the incident could have been planned like the earlier Mukden incident but Jim Huffman notes this notion has been widely rejected by scholars. The first shot remains attributed to an unplanned moment of fear according to many accounts involving a low-ranking soldier.