Edit: post was intended to be about the long WWII era (post WWI era to immediate aftermath of WWII) rather than the cold war though I'd be interested in the cold war history if people have that available.
Are you talking about WW2, or the Cold War?
China was not considered for the most part because it was essentially in the middle of political turmoil and later a Japanese invasion. Japan however was definitely a consideration and there was even a full-scale border battle fought against them in the 30s in the form of Khalkin Gol.
However, the threat from Japan was greatly reduced after the Soviets won an absolutely crushing victory in that battle - the Japanese attackers were essentially surrounded and wiped out; and the Imperial Japanese Army was made aware of how weak their army really was when faced with modern tanks and artillery.
Indeed, this was likely the reason why Japan never declared war on the Soviet Union after Pearl Harbor despite Germany declaring war on the United States. Japan's meek acceptance of the Soviet position in the Far East was such that they did not even blockade Vladivostok throughout the war and allowed American Lend-Lease shipments (on Soviet-flagged ships) to be delivered to the Soviets via this port to help fight the Nazis! Their only condition was that Lend-Lease supplies delivered through Vladivostok should not be directly military in nature; but nonetheless the shipments included huge stocks of grain that saved many Soviet citizens from starvation and allowed them to keep working on armaments..