The Battle of Mount Song (松山战役) in 1944 was the largest campaign in southwestern China during the Second World War. Chinese Nationalist forces aimed to retake control over the Burma Road. The Japanese were losing the war in Burma and aimed to block off the highway for as long as they could. They used slave labor from Thailand and Burma to construct a series of tunnels and bunkers in order to turn the mountain into a fortress. The Chinese forces were unaware of the depth of the Japanese defenses, and their underestimation led to heavy casualties. Chinese artillery strikes and US bombing runs had little effect against Japanese forces underground. Japan also set up a series of hidden pillboxes to ambush the Chinese forces. After three months of bloody battle, the Chinese forces finally retook Mount Song through the use high-powered explosives and flamethrowers and the Burma Road could be used once again.

• http://en.wikipedia.org/wiki/Battle of Mount Song