By 1937, Japan controlled Manchuria and was ready to move deeper into China. The Marco Polo Bridge Incident on 7 July 1937 provoked full-scale war between China and Japan. The Nationalist and Communist Chinese suspended their civil war to form a nominal alliance against Japan, and the Soviet Union quickly lent support by providing large amount of materiel to Chinese troops. In August 1937, Generalissimo Chiang Kai-shek deployed his best army to fight about 300,000 Japanese troops in Shanghai, but, after three months of fighting, Shanghai fell. The Japanese continued to push the Chinese forces back, capturing the capital Nanking in December 1937 and committed which was known as Nanking Massacre. In March 1938, Nationalist forces won their first victory at Taierzhuang. but then the city of Xuzhou was taken by Japanese in May. In June 1938, Japan deployed about 350,000 troops to invade Wuhan and captured it in October. The Japanese achieved major military victories, but world opinion—in particular in the United States—condemned Japan, especially after the Panay Incident.

What incident occured on July 7, 1937?