In the Fall of 1941 German tanks were rolling across the Soviet Union, on their way to a near conquest of that huge country. By that winter they had reached the gates of Moscow, and had captured most of the industrialized section of the USSR. Yet their allies, the Japanese, stood by in the East, with substantial forces in Manchuria and elsewhere in continental Asia, doing nothing to attack the USSR. Instead the Empire of Japan looked to the south and to the vast Pacific, and decided to attack the forces of Britain and America.
In the strategic simulation game "Axis and Allies", the forces of Japan and Germany have an excellent chance of achieving victory by jointly attacking the USSR. This would have been especially true in late '41 when the United States was not yet actively at war with either country. So why didn't Japan attack towards the west in 1941?
One factor which is rarely mentioned in the history books is the fact that in the summer of 1939 they tried. In May of that year, Japanese troops crossed the Mongolian-Manchurian border, and had taken up positions. In what was to become a turning point in his career, Georgi K. Zhukov, a Soviet officer who had escaped from Stalin's devastating purges of the military in 1937-38, masterminded and commanded the defense and counter-attack which was to rout the Japanese forces in the summer of 1939, in the battle of Halhin Gol.
Zhukov proposed to hold the east bank of the Halhin Gol river with existing Soviet and Mongolian troops (the 57th Special Corps), and to put together a counter-attack, including reinforcements of three infantry divisions, more artillery, a tank brigade, and significant aviation support. In the attack of Aug 20th, Zhukov was able to completely encircle two Japanese infantry divisions, support troops, and a tank brigade. The Japanese attempted a breakout on the 27th, but failed. The battle was over by Aug 31st with the near-complete destruction of the Manchukuo 23rd. By mid-September the Japanese had lost a total of 61,000 men, and were in full retreat. The Japanese then concluded a non-aggression pact with the USSR and reaffirmed the pre-existing borders. (Soviet casualties were around 18,000 men, of whom about 7,000 were killed).
Zhukov was promoted to General of the Army, given the gold star of a Hero of the Soviet Union, and assigned to command the Kiev Military District. He was later to become the most famous and important soviet general of World War II, saving Moscow and Leningrad in the winter of 1941-42, planning the grand strategy of the counter-attack at Stalingrad in late 1942 and the strategy at Kursk in 1943, and finally taking Berlin in 1945.
The most comprehensive book on the subject in English is "The Nomonhan Incident" by Dr. Alvin Coox of San Diego State University, and can be found at Amazon.com
Note the recent thesis at East Tennessee State Univ.: Bradley S. Thomas. "Japan and Russia at Nomonhan, 1939: A Battle Which Altered Japanese Aims in Siberia and China", 1994 (Antkiewicz, Fritz, Baxter), (Masters in History).
Also, consider the May-Jun '93 edition of Armor Magazine, which contains an article "Nomonhan: Prelude to World War II"