The Hundred Regiments Offensive(August 20, 1940 - December 5, 1940)was a major campaign of the Communist Party of China's Red Army commanded by Peng Dehuai against the Imperial Japanese Army in Central China.
Between 1939 and 1940 the Japanese occupiers launched more than 109 small campaigns involving around 1000 combatants each and 10 large campaigns of 10000 men each to wipe out Communist guerrillas in the Hebei and Shantung plains. In addition, Wang Jingwei's anti-Communist puppet government had its offensive against the CCP guerillas. It was out of these circumstances that the CCP planned to stage a great offensive.
The Japanese North China Area Army estimated the strength of communist regulars to be about 88,000 in December 1939. Two years later they revised the estimate to 140,000. On the eve of the battle the Communist forces grew to 400,000 men strong, in 115 regiments. The extraordinary success and expansion of the Eighth Route Army against the Japanese led Zhu De and the rest of the military leadership to hope that they could engage the Japanese army and win. Mao Zedong argued that the war against Japan would be protracted, and that communist strategy should emphasize guerrilla warfare, political mobilisation and the building up of base areas.
Nevertheless, by 1940 growth was so impressive that Zhu De ordered a coordinated offensive by most of the communist regulars (46 regiments from the 115th Division, 47 from the 129th, and 22 from the 120th) against the Japanese-held cities and the railway lines linking them. From 20 August to 10 September communist forces attacked the railway line that separated the communist base areas, chiefly those from Dezhou to Shijiazhuang in Hebei, Shijiazhuang to Taiyuan in central Shanxi, and Taiyuan to Datong in northern Shanxi. They succeeded in blowing up bridges and tunnels and ripping up track, and went on for the rest of September to attack Japanese garrisons frontally, taking excessive casualties (22,000 regulars, compared to Japanese losses of 3000 or 4000). In all, about six hundred miles of railways were destroyed and the Chingching coal mine, which was important to the Japanese war industry, was rendered inoperative for six months. It was the greatest victory the CCP fought and won during the war.
However, from October to December the Japanese responded in force, reasserting control of railway lines and conducting aggressive "mopping up operations" in the rural areas around them. When General Okamura Yasuji took command of the North China Area Army in the summer, the new approach was "Three All" meaning kill all, burn all, and destroy all in those areas containing communist forces. The population of the communist base areas dropped dramatically and communist operations were severely limited, with CCP forces reduced to 300,000 men. Communist control also reduced to 10 out of 437 counties in North China. Mao used the subsequent rectification campaign to reassert his personal authority over the party and over military strategy, and this meant the abandonment of any serious communist challenge to the Japanese position in North China for the rest of the war.