Wikipedia: The Battle of Wuhan, also known to the Chinese as the Defense of Wuhan, and to the Japanese as the Capture of Wuhan, was a large-scale battle of the Second Sino-Japanese War. Engagements took place across vast areas of Anhui, Henan, Jiangxi, Zhejiang, and Hubei provinces over a period of four and a half months. This battle was the longest, largest and arguably the most significant battle in the early stages of the Second Sino-Japanese War. More than one million National Revolutionary Army troops from the Fifth and Ninth War Zone were put under the direct command of Chiang Kai-shek, defending Wuhan from the Central China Area Army of the Imperial Japanese Army led by Shunroku Hata. Chinese forces were also supported by the Soviet Volunteer Group, a group of volunteer pilots from the Soviet Air Forces.
Although the battle ended with the eventual capture of Wuhan by the Japanese forces, it resulted in heavy casualties on both sides, as high as 1.2 million combined by some estimates. With the Japanese suffering their heaviest losses of the war, they decided to divert their attention to the north, prolonging the war until the attack on Pearl Harbor.