Short answers, pulled from the story.
Hokushin-ron emerged from the First Sino-Japanese War during the 1890s as an Imperial National Defence guideline. Field Marshal Prince Yamagata Aritomo traced lines of a defensive strategy against Russia in February 1907 that included this doctrine.
Supporters known as the Strike North Group viewed Manchuria and Siberia as Japan's sphere of interest. General Kenkichi Ueda believed Japan's main enemy was communism and remained adamant in his support of the Hokushin-ron policy throughout the period.
The Mukden Incident occurred in 1931 after insubordination by rogue military personnel led to the invasion of Manchuria. A puppet state named Manchukuo formed in Northeast China and Inner Mongolia later that same year under a form of constitutional monarchy.
Aggressive actions initiated by Japanese staff led to the disastrous Battles of Khalkhin Gol in 1939 which resulted in heavy casualties for the Kwantung Army. Any farther expansion northwards into Siberia proved impossible because of Soviet superiority in numbers and armour.
Japan and the USSR signed the Soviet, Japanese Neutrality Pact in April 1941. This agreement freed Japan for preparations for the Pacific War while Japan did not join Nazi Germany's invasion of the Soviet Union launched in June 1941.