Short answers, pulled from the story.
Kantokuen was a planned World War II Japanese military campaign designed to seize virtually the entire Soviet Far East up to Lake Baikal. The operational plans were progressively increased in scope from small-scale operations to gigantic multistage campaigns between 1938 and 1941.
The Kwantung Army and the Operations Bureau of the Japanese Army General Staff cooperated on related contingencies under Operational Plan No. 8 or Hachi-Go. Major General Shin'ichi Tanaka served as Chief of the Operations Bureau and pushed for quick decisions regarding the northern expansion strategy.
Japan cancelled the Kantokuen invasion because the United States enacted a total oil embargo on the 1st of August 1941 which cut off 80% of Japan's oil supply. This economic sanction proved the final nail in the coffin of the operation and forced the Army General Staff to prioritize seizure of resource-rich countries in Southeast Asia instead.
Concept B examined striking into the vast steppe between Great Khingan Mountains and Lake Baikal hoping to quickly cut the Trans-Siberian Railway. Such movement would isolate the whole region from European Russia and doom remaining defenders to defeat in detail.
Kantokuen plans called for committing up to 50 divisions delivered incrementally from China and Home Islands against expected 60 Soviet divisions. The final buildup halted with only 16 divisions standing guard while all reinforcements totaled 463,000 men by the time operations ceased.