Short answers, pulled from the story.
Kantokuen was an operational plan created by the General Staff of the Imperial Japanese Army for the invasion and occupation of the Russian Far East, drawn up to capitalize on the outbreak of the Soviet-German War in June 1941. It would have involved seven Japanese armies and a major portion of Japan's naval and air forces, making it the largest combined arms operation in Japanese history up to that point.
Emperor Hirohito gave partial approval to the Kantokuen buildup on the 7th of July 1941, after General Hajime Sugiyama visited the Imperial Palace to request his sanction and promised that the Kwantung Army would not attack on its own initiative.
Kantokuen fell out of favor at Imperial General Headquarters as preparations for a Southeast Asia offensive grew, the Second Sino-Japanese War kept draining resources, and prospects for a swift German victory over the Soviets dimmed. It was finally abandoned after the United States and its allies imposed an oil embargo and other economic sanctions in August 1941.
The plan involved seven Japanese armies, backed by a major portion of the empire's naval and air forces.
Genrikh Lyushkov was the chief of the Far Eastern Department of the Soviet NKVD who defected to Japan on the 13th of June 1938, crossing into Manchuria with a wealth of secret documents on Soviet military strength and dispositions. His defection was a major intelligence coup for Japan, and he continued working against the Soviet Union until he disappeared during the Soviet invasion of Manchuria in August 1945.
The presence of large Japanese forces in Manchuria forced the Soviets, who had long anticipated an attack from that direction, to retain considerable military resources in Siberia throughout World War II.