July 7, 1937: The Marco Polo Bridge Incident begins.
The battle on the Marco Polo (or Luguo) Bridge, located kilometers outside Beijing, then called Beiping, marked the beginning of the full-scale invasion of China by the Japanese Empire and the beginning of the Second Sino-Japanese War, the largest war in Asia of the 20th century. It coincided with and eventually merged with World War II, becoming a part of the larger conflict in the Pacific and much of Asia.
In 1931, Japan seized the northeastern portion of China called Manchuria and installed there a puppet state under the name “Manchukuo"; under the governance of the former Chinese emperor Puyi and the direction of the Japanese Empire, Manchukuo was effectively detached from the rest of China. Over the next few years, Japan continued to industrialize the region and build its influence in the regions surrounding Manchukuo, expanding its territory and stationing troops along the railways leading to Beijing, until finally, a skirmish between Japanese and Chinese troops on the Marco Polo Bridge over a lost Japanese soldier (later determined to have wandered off to relieve himself) heightened tensions between the nations - and eventually, escalated into war. The incident has often been interpreted as both the result of a series of misunderstandings and accidents to the fault of both the Japanese and Chinese, and as an incident intentionally staged by the Japanese as a pretext for a full-blown invasion of China.
The Japanese forces outmatched the KMT National Revolutionary Army, then also entangled in civil war with Communist forces, in terms of industrial strength, technology, and training, and quickly overwhelmed them, occupying Beijing and taking Shanghai after a bloody bout of urban warfare in August to November 1937. A war marked by great loss of life (both military and civilian), unspeakable atrocities, and strategic stalemates began at Marco Polo Bridge on July 7, 1937, and it would not end for eight years.