On July 7, 1937, at the Marco Polo Bridge, the Japanese Kwantung army stationed there used explosions heard on the Chinese side of Manchuria as a pretext for invasion. The invasion led to a large scale war approved by the Emperor and called a "holy war" (Seisen) in Imperial propaganda.
At the time, China was divided internally between the Communist Party of China (CPC) which was under the leadership of Mao Zedong and the Nationalist government of China, the Kuomintang (KMT) under the leadership of Chiang Kai-shek.
The years of 1937–38 were a time of rapid and remarkable success by the Japanese, who had a number of advantages over the Chinese army. While the Japanese army possessed a smaller force of armour and artillery than many Western powers, it was far ahead of China in this respect, and was also in command of the world's third largest navy with 2,700 watercraft at its disposal.
By the end of July 1937, the Japanese had slaughtered the elite 29th Army at Kupeikou[citation needed] and soon captured Beijing. From there, the Japanese advanced down south through the major railway lines (Peiping-Suiyan, Peiping-Hankow, and Tientsin-Pukow). These were easily conquered by the superior Japanese army.
By October, Chiang Kai-shek's best armies had been defeated at Shanghai. By the end of the year, the Chinese capital at Nanjing had also been seized. The use of brutal scorched earth tactics by both sides, the Chinese as in 1938 Yellow River flood and later by the Japanese with the Three Alls Policy, "kill all, burn all, loot all", initiated in 1940, claimed millions of lives. The Chinese nationalists resorted to massive civilian guerrilla tactics, which fatigued and frustrated Japanese forces. Countless Chinese civilians were executed on the suspicion of being resistance fighters. Japanese war crimes at Nanking and other sites in China and Manchukuo have been well documented.
On December 13, 1937, the Imperial Japanese Army, following the capture of Nanjing, Japanese soldiers began the Nanjing Massacre (sometimes called the "Rape of Nanking"), which resulted in a massive number of civilian deaths including infants and elderly, and the large-scale rape of Chinese women. The exact number of casualties is an issue of fierce debate between Chinese and Japanese historians (refer to the Nanjing Massacre article for more details).
By 1939, the Japanese war effort had become a stalemate. The Japanese army had seized most of the vital cities in China, including Shanghai, Nanjing, Beijing, and Wuhan. The Nationalists and the Communists, however, fought on from Chongqing and Yenan, respectively.