Short answers, pulled from the story.
The Marco Polo Bridge incident was a three-day battle that began on the 7th of July 1937 between the Chinese 29th Army and the Imperial Japanese Army near Wanping, 16.4 km southwest of Beijing. It is regarded as the start of the Second Sino-Japanese War because, unlike earlier skirmishes, the tensions following it escalated into full-scale conflict rather than subsiding.
The immediate trigger was the reported absence of a Japanese soldier, Private Shimura Kikujiro, during night exercises on the 7th of July 1937. Japanese officers demanded entry into Wanping to search for him; Chinese forces refused. Shimura had already returned to his unit before fighting broke out. The exact cause of the first shot remains disputed.
The Marco Polo Bridge, also called Lugou Bridge, spans the Yongding River and is located about 16.4 km southwest of Beijing near the walled town of Wanping. It is an eleven-arch granite structure first built under the Jin dynasty and restored in 1698 under the Kangxi Emperor.
On the Chinese side, the main figures were General Song Zheyuan, commanding the 29th Army; acting commander General Qin Dechun; and Colonel Ji Xingwen, who led roughly 100 defenders at the bridge. On the Japanese side, Lieutenant General Kanichiro Tashiro commanded the China Garrison Army, while Major General Masakazu Kawabe commanded the China Garrison Infantry Brigade.
Historians have widely rejected the idea that Japan staged the incident. According to Jim Huffman, Japan was more focused on the Soviet threat at the time. Historian Ikuhiko Hata has suggested the Chinese Communist Party may have provoked the clash, but he considers the accidental shot hypothesis more likely: that a low-ranking Chinese soldier fired in an unplanned moment of fear.
After a verbal ceasefire collapsed, Japanese reinforcements poured in and by the 20th of July 1937 Japanese strength in the Beiping-Tianjin area exceeded 180,000 personnel. General Song Zheyuan was defeated on 27-the 28th of July, and Japan captured Beiping and the Taku Forts at Tianjin on 29 and the 30th of July respectively, concluding the Battle of Beiping-Tianjin after 24 days of combat.