In ww2 the USSR was at war with the axis and only shared a land border on Sakhalin, Korea (and I think Manchuria was a puppet of Japan at the time. How come there was absolutely no invasion from the USSR or Japan? Thanks.
Your premise is incorrect, near the end of the war starting in August 1945 the Red Army launched a massive invasion of Japanese holdings on the Asian continent as well as in Sakhalin and the Kuril Islands. Manchuria, some of Inner Mongolia, and most notably the Northern part of Korea were occupied by the Soviets during this campaign which hastened Japan’s unconditional surrender.