Short answers, pulled from the story.
The Kwantung Garrison began as a small security force of 14,000 men in 1906. It guarded the Kwantung Leased Territory on the Liaodong Peninsula and protected the South Manchurian Railway Zone.
By the 1920s, the Kwantung Army became a stronghold for the radical Imperial Way Faction. Senior leaders openly advocated violent overthrow of Japan's civilian government and plotted assassinations against warlords like Zhang Zuolin during the Huanggutun Incident of 1928.
Conspirators within the Kwantung Army engineered the Mukden Incident in 1931 to justify an invasion of Manchuria. By 1932, the unit established the puppet state of Manchukuo in Northeast China and Inner Mongolia with a commander holding authority equivalent to a Governor-General.
Soviet forces challenged the Kwantung Army during border conflicts starting in 1938. The Battle of Lake Khasan ended in a stalemate while the decisive Battle of Nomonhan resulted in heavy casualties for Japan one year later.
By 1945, the Kwantung Army numbered 713,000 personnel across 31 infantry divisions but quality had collapsed as best units were stripped away. The force possessed 1,155 light tanks, 5,360 guns, and 1,800 aircraft that were woefully outdated compared to modern standards.
Dr. Shirō Ishii directed human experimentation programs using Chinese, American, and Russian civilians within secret installations housing chemical and biological weapons. Arrested by American occupation authorities, Ishii and 20,000 members received immunity from prosecution in exchange for providing germ warfare data based on unethical experiments.