The Japanese Empire and the Soviet Union has just ceased hostilities in 1939, after the Soviet Union destroyed the Japanese Sixth Army at Khalkhin Gol / Nomonhan.

Furthermore the Soviet Union maintained a large army in the Far East at all times during the war, easily enough to seriously hamper any Japanese attack.

Order of Battle, 22 June 1941.

Source: David Glantz, Stumbling Colossus.

Another point seems to have been to that the Japanese Imperial Navy Staff wanted to gain honour and glory, and they won the internal power struggle.

But

"the Japanese humiliation at Nomonhan was an important factor in its decision to pursue the southern option that was the attack on Southeast Asia to secure the oil of the Dutch East Indies, without which it would not survive the pending U.S. oil embargo. This is a conclusion that many readers may justifiably question. Not only were there strong voices in Japanese military circles who were still eager to follow the northern option--an attack to seize Soviet Siberia--but preparations were actually taken to accomplish that in 1941, with a buildup of 700,000 men in the Kwantung Army. Goldman clearly shows that it was the defeat of the Germans in front of Moscow, not the defeat at Nonomhan, that convinced the Japanese not to invade Siberia. There were those in favor of the southern option who reminded their colleagues of Nomonhan, but this may be seen more as a tactic than a conviction. Their real conviction was the critical need for a secure source of petroleum."

Source/quote: Criticism on Stuart D. Goldman. Nomonhan, 1939: The Red Army's Victory That Shaped World War II.

British lend-lease had started at that point, Churchill was hardly inactive towards the Soviet Unions plight by October 1941.