The Chinese and Japanese had been fighting each other for several decades before the beginning of World War II. China and Japan fought over Korea before the turn of the 20th century, and Japan became a more expansionist power in the region with an aggressive colonization program. Multiple incidents in the 1930s led the two powers to begin officially fighting the Second Sino-Japanese War in July 1937; the fighting continued until the end of World War II in 1945.
First Sino-Japanese War
The Chinese and Japanese fought each other for control of Korea in the First Sino-Japanese War (1894-1895). In 1875, Japan had forced Korea to declare itself independent of China and open up to Japanese trade. Japan was interested in Korea due to its strategic location and valuable natural resources, such as iron and coal.
On September 18, 1931, the Japanese army destroyed a section of track on a Japanese-owned railway near the Chinese city of Mukden. The Japanese government blamed the explosion on Chinese nationalists, using the incident as a pretext to invade Manchuria. The Japanese took over the resource-rich region in a matter of months, declaring the area to be the new independent state of Manchukuo. But, according to the U.S. Department of State's Office of the Historian, the new state was not really independent and the local Japanese army was actually in control of the area.
Second Sino-Japanese War
A small battle on the Marco Polo Bridge near Beijing between the Republic of China's army and the Japanese army started the Second Sino-Japanese War in 1937. The first major battle in the war was the Battle of Shanghai, which ended in November 1937 and had industrial damages estimated at over 560 million yuan. China received aid from Western democracies such as Britain, France and the United States.
Rape of Nanjing
Japanese war crimes against China increased Chinese animosity against Japan in the years leading up to World War II. The most famous war crime was the Rape of Nanjing, when the Japanese military killed thousands of Chinese civilians after they captured the city of Nanjing on December 13, 1937. According to a 2002 article published in the "Electronic Journal of Contemporary Japanese Studies,'' the scholarly estimates on total deaths in the Rape of Nanjing range from several thousand to 200,000 or more.