Had the Japanese stopped with Manchukuo, which had a significant supply of important raw materials & space for colonists there would have been no war with the USA. Assuming that Europe would be involved in war, this would open the way for Japan to become economically dominant in China, and had the KMT made a deal with the Japanese it is entirely possible the KMT could have beaten or contained the communists. By the end of the war in Europe some time in the 1940s Japan would have been in a position to be the economically dominant power in China, even with a Chinese government that allowed fairly open trade (thus mollifying the US). This scenario, however, was not good enough for the elements in Japan who wanted the physical/formal empire structure that existed in Korea, Manchukuo, and Taiwan extended to a large chunk of China.
Attacking north to Siberia, even without the lessons of Khalkin Gol, was simply not an option. In 1941 all of the natural resources of Siberia were either totally unknown of not obtainable with then available technology - and there was precious little infrastructure to move said resources anywhere. On the other hand, SE Asia had all of the raw materials Japan needed, and well developed extraction and transportation infrastructure fo these resources. Also, don't forget that in the 5-8 years before PH the USA had been progressively strangling Japan economically so by summer 1941 Japan could not purchase the raw materials it needed, even had the British or Dutch been willing to sell them oil for example.
The choice was very simple: end the war in China or seize what you need to fight the war & maintain your economy. For many reasons, taking the latter course will necessitate war with the USA. Sadly, for many reasons, choice (b) was made. Even after Nanjing & other atrocities had Japan ended the war and "retreated" to Manchukuo a modus vivendi with the USA could have been worked out on terms that the Japanese would have found favorable.