Short answers, pulled from the story.
Hokushin-ron was a political doctrine of the Empire of Japan before World War II holding that Manchuria and Siberia offered Japan greater economic and territorial value than any other direction of expansion. Its supporters were sometimes called the Strike North Group.
Japan abandoned Hokushin-ron in 1939 after the Imperial Japanese Army suffered a military defeat at the Battles of Khalkhin Gol, known in Japan as the Nomonhan Incident. Soviet superiority in numbers and armor showed that further expansion north into Siberia was impossible.
Nanshin-ron replaced Hokushin-ron, shifting Japan's focus to Southeast Asia and the Pacific islands. The new policy aimed to acquire the resources of European colonies and to neutralize the threat of Western military forces in the Pacific.
Hokushin-ron was widely supported within the Imperial Japanese Army, including War Minister Sadao Araki, General Kenkichi Ueda, and General Yukio Kasahara. The radical Imperial Way Faction, or Kōdōha, was especially strongly in favor of the policy.
Hokushin-ron called for Japan to seize Manchuria to gain a direct land border with the Soviet Union. Insubordination by Japanese military personnel in the Kwantung Army caused the 1931 Mukden Incident, which gave Japan its pretext to invade.
Japan and the Soviet Union signed the Soviet-Japanese Neutrality Pact in April 1941, freeing Japan to prepare for the Pacific War instead. Japan did not engage the Soviet Union militarily again until the Soviets declared war in August 1945.