The Second Sino-Japanese War (1937-1945), which was fought between China and Japan, was the largest war in Asia of the 20th century. The tension that led to it had its roots in the First Sino-Japanese War, which took place nearly three decades previously. In the first Sino-Japanese war, Japan captured Taiwan and Korea, which had previously been under Chinese control. Years of conflicting national policies put these two great powers at odds — Japan's imperialistic dominance put an embittered nationalistic China on the defensive, and the instability in the region contributed to hostilities. Also known as the War of Resistance Against Japan to the Chinese, and as the Japan-China War to Japan, the Second Sino-Japanese War began with several years of scattered battles and incidents that eventually led to a full blown war that would not end until the culmination of World War II.
After the First Sino-Japanese War, a railway in Korea that was used to supply Japanese troops also ran through parts of Manchuria, which was a source of some of these supplies. In 1931, some Japanese troops attacked Chinese troops in the area, using a fake attack on the railway as an excuse. The fighting spread, and before long Japan controlled a large portion of Manchuria. Japan renamed the area Manchukuo and put a token Chinese government in place, but the behind the scenes, the Japanese army retained power. Between 1933 and 1935, China established two demilitarized zones around Manchukuo, a sign of weakening resistance.
After the Japanese seizure of Manchukuo, Japan and China engaged in occasional battles with each other, but the fighting spread in 1937 after some Chinese troops attacked Japanese troops near Peking. Most historians agree that the Second Sino-Japanese War began in earnest at this time. Soon after that, Japan quickly captured the most important coastal cities and Chinese troops retreated, largely due to their supply deficiencies, poor training, and military corruption.
Later that year, the Japanese Army captured and sacked Nanjing in one of the important victories of the Second Sino-Japanese War. The Japanese government feared that news of the thefts, rapes, and murders of Chinese citizens and visiting Westerners in Nanjing would spread because American, Dutch, and British soldiers were nearby assisting in an unrelated refugee evacuation. Japanese pilots were ordered to attack, resulting in the deaths of two of the Western soldiers. This shocked foreigners, but the news of what happened in Nanjing was suppressed in order to prevent demands for declaration of war against Japan.
Not long after the capture of Nanjing, the invading Japanese Army slowed down due to the sheer size and available manpower of China and its lack of infrastructure. A stalemate ensued and lasted until 1941. The occupation was very difficult for the Chinese, as conditions had already become uneasy due to political instability. Many suffered from food shortages, and the Nationalist and Communist leaders were more involved with fighting each other than fighting off the Japanese invasion.
In 1941, when China entered World War II, the Second Sino-Japanese War was subsumed into the larger World War II effort. The United States and the Soviet Union aided China in the war against Japan. The Second Sino-Japanese War ended in 1945 as a part of Japan's surrender at the end of World War II, during which Japan returned control of Manchuria, Taiwan, and the Pescadores Islands to China. Because the Second Sino-Japanese War greatly weakened the Nationalists, the Communists later took over China in 1949.