1939 was proving to be a dismal year for the Japanese. While they had conquered huge swathes of Northern and Eastern China, they were bogged down in a bloody stalemate in the Center and South of the country. The Japanese also lost heavily in two major battles: With the USSR at Khalkin Gol(September 1939); and with the Nationalist Chinese(KMT(Kuomintang)) at Changsha(October 1939). The Japanese were also engaged in a bloody counterinsurgency campaign with the Chinese Communists(CCP) for control of Northern China. And on top of all this, massive Soviet & Western Allied(USA, Great Britain & France) support for Generalissimo Jiang Jie Shi(Chang Kai Shek) had kept the KMT armies in fit fighting form(more or less) and kept armed resistance versus the Japanese invaders strong.
For the Japanese, 2 years of war in China had cost them dearly not only in men and materiel but also the Japanese economy was in a rapid downward spiral. The anticipated economic exploitation of the conquered Chinese territories was proving to be an astounding failure due to massive Chinese guerilla resistance. While the Japanese were winning most battles, they were stalled and final victory was nowhere in sight. Negotiations with the KMT Chinese went nowhere and demands for the Allied powers to end support for the KMT fell on deaf ears.
Desperate to bring about final victory(or at least a negotiated settlement from the KMT). On November 15, 1939, the Japanese landed a full Japanese division reinforced by three brigades(total of around 50,000 men)(21st Corps) commanded by General Kinichi Imamura on the coast of the Chinese province of Guangxi. With huge air and naval support, the Japanese within 9 days managed to capture the important provincial capital of Nanning with little resistance.
The Japanese objective was to capture Nanning as this city was an important transportation nexus for supplies coming from the French Indochina border. Due to unstable relations with the British resulting in the closing of the Burma road, Generalissimo Jiang Jie Shi was forced to rely on supplies coming in from French Indochina to keep his armies alive. The capture of Nanning cut this vital supply line. Worse still, the Japanese advanced north beyond Nanning and by December 4, 1939 had captured the important Kunlun Pass.
Kunlun Pass was an important strategic position. Japanese capture threatened KMT rear areas and most importantly the KMT Chinese wartime capital of Chungking. In desperation, Generalissimo Jiang Jie Shi sent one of his best commanders, General Bai Chongxi(himself a Guangxi native and was one of the architects of the Chinese victories at Taierzhuang 1938 and Changsha 1939) along with 50,000 of his most elite German trained troops(3 divisions, 5th Corps commanded by General Tu Yu Li Ming) along with around 100 Russian T-26 tanks and Italian CV-33 tankettes to the task of recapturing Kunlun Pass from the Japanese.
Defending Kunlun Pass were 5,000 men of the elite Japanese 21st Infantry regiment/21st Infantry Brigade(“Unbreakable Sword”)/5th division(“Koi”) in fortified mountain positions amply backed up by air and artillery support commanded by Major General Masao Nakamura.
The result would be one of the most famous battles of the Second Sino Japanese war and the largest massed commitment of Chinese armor in the whole war.