After two years of near stalemate, Lieutenant General Seishiro Itagaki led the Japanese 101st and 106th Divisions to Changsha, the capital of Hunan Province, while the 3rd, 5th, 6th, 13th, and 33rd Divisions invaded the general northern Hunan region to assert additional pressure against the city. The Japanese forces totaled 100,000 men, commanded from the field by Yasuji Okamura.

The Chinese defenses were led by Nationalist Generals Chen Cheng and Xue Yue. Outnumbering the Japanese nearly two-to-one, they were able to entertain the plans for a counteroffensive as soon the Japanese attack launched on 17 Sep 1939. The two forces met in Jiangxi Province to the east of Hunan, resulting in a quick Chinese victory that pushed the westward marching Japanese invasion force back.

On 19 Sep, the Japanese launched a poison gas attack, against the laws of war set by the Geneva Protocol, on the Chinese forces along the Sinchiang River, driving the Chinese forces out by 23 Sep.

After nearly a week of fierce fighting, the Japanese reached the outskirts of Changsha on 29 Sep, but by this time the Japanese forces were already weakened by heavy casualties estimated at over 40,000, with a bulk being fatalities. After constant harassment, the over-stretched supply lines were also in danger of being completely severed. The remnants of the Japanese forces fled north by 6 Oct. The 16 Oct 1939 issue of Time Magazine reported that

“the Chinese turned around and, with a fury they have never shown before, lashed the Japanese back and back…. the Chinese destroyed every rail line, every road. The Japanese blithely advanced over this torn-up area until they were in the worst military position known to man: on a thin front without communications behind. That was when the Chinese struck. The Japanese had nothing to do but run.”This battle represented the first major defeat of the Japanese Army at a major Chinese city since the start of the Second Sino-Japanese War in Jul 1937.

Thanks to WWII Database and C.Peter Chen for the detailed write up which can be found here.