The staged incident on July 7, 1937 that would lead to the all-out invasion of China by the Japanese.
A Japanese company was on a "training exercise," near the Marco Polo Bridge, near the garrison of Wanping in the outskirts of Beijing. Citing a missing soldier, the company commander demanded entry into the garrison to search. He was refused entry, and subsequently fired upon the garrison. Despite seeming like a minor scuffle, it ended up leading to full scale war.