The Soviets had already walloped the IJA once with one-arm tied behind their back. See the Battle of Khalkhin Gol
Colonel Masanobu Tsuji was a notorious Army militant and war criminal. Despite his relatively modest rank, he was remarkably influential (reportedly charismatic), and was often present at Imperial War Council meetings, where he opposed any peace moves whatever. However, he had been present in Manchuria during the clash with the Soviets, and the beating the Japanese got was enough to make him opposed to any further trying-on with them.
Furthermore, there wasn't oil exploration in Siberia at the time. They couldn't support their empire on the promise of future oil trade with Germany in the event the Soviets were beaten. It was needed almost immediately or the war effort in China was going to collapse.
The oil that was within reach was in the Dutch East Indies.
And in Brunei, then a British protectorate. During the war, IJN ships sometimes used unrefined Brunei crude for fuel (it's fairly light). IIRC, some of the Japanese Navy based at Brunei for that purpose, to save tanker capacity.
Securing it means taking the Phillipines, or else the Asiatic Fleet or land based planes could just sit astride the sea lanes and prevent the oil from reaching the home islands, because there was no way in Hell that the US would let them get away with an unprovoked war of aggression against the Dutch.
And British - but that was the Japanese assumption, and I don't know that it is true. If Japan invaded Malay, Sarawak/Brunei, and the Dutch East Indies, but did not attack the Philippines or any other U.S. territory,
would the U.S. go to war? Seriously, would the Congress of 1941 declare war over actions on the other side of the world, not involving the U.S.? I doubt it.
This necessitates crippling the Pacific Fleet, hence the attack on Pearl Harbor.
It means war with the U.S., but it was Yamamoto who insisted on the Pearl Harbor strike, over the objections of the Navy plannng staff. The staff idea (which had been the Japanese plan for years) was to seize the Philippines and Guam, and wait for the U.S. Pacific Fleet to sail west to the rescue. Japanese airpower, submarines, and small craft would harass and attrit the U.S. fleet on its long voyage, and then the Japanese fleet would meet and destroy the Yankees in "one great battle".