Short answers, pulled from the story.
The Kwantung Army was a general army of the Imperial Japanese Army that operated from 1919 to 1945. It began as a garrison force protecting the Kwantung Leased Territory and the South Manchurian Railway Zone, then expanded into a force that engineered Japan's occupation of Manchuria, administered the puppet state of Manchukuo, and fought in the Second Sino-Japanese War.
The Kwantung Army was the most prestigious command in the Imperial Japanese Army, and many of its officers rose to high positions in Japan's military and civilian government. Notable figures who served in its ranks include Hideki Tojo and Seishiro Itagaki, both of whom later held senior roles in the Japanese state.
In 1931, Kwantung Army officers engineered the Mukden Incident as a pretext for invading Manchuria, defying direct orders from Tokyo. By 1932, the army had established Manchukuo as a Japanese puppet state, with former Qing emperor Puyi installed as nominal ruler while the Kwantung Army's commanding officer held authority to override any of Puyi's decisions.
The Soviet Union launched its invasion of Manchuria on the 9th of August 1945, swiftly defeating the Kwantung Army. General Otozō Yamada ordered the surrender on the 16th of August 1945, one day after Emperor Hirohito announced Japan's capitulation. Over 500,000 Japanese prisoners were subsequently sent to Soviet labor camps in Siberia, the Russian Far East, and Mongolia.
Unit 731 was a covert biological and chemical weapons program that the Kwantung Army sponsored in Manchuria. It conducted unethical human experimentation on Chinese, American, and Russian civilians and prisoners of war under the direction of Dr. Shiro Ishii. After Japan's surrender, the approximately 20,000 members of Unit 731 received immunity from war crimes prosecution in exchange for providing germ warfare data to American authorities.
Several former Kwantung Army leaders were sentenced to death at the 1948 International Military Tribunal for the Far East in Tokyo, including Seishiro Itagaki, Hideki Tojo, Kenji Doihara, Iwane Matsui, and Akira Muto. Twelve members of Unit 731 were separately convicted at the Khabarovsk War Crime Trials held under Soviet jurisdiction.