I’ve been glancing at this thread on-and-off for a while. I don’t have any particular opinion on the NAP options being discussed, but here are a few bits of background information in case they’re of any use. I’d also like to put forward a few arguments to suggest that an outbreak of war between Japan and the USSR in the early 1940s wasn’t a flat-out impossibility, and to suggest that any game scenario featuring such an outbreak might plausibly use Mongolia or Manchuria as its flashpoint.
In the decades before WWII, there was no love lost between Japan and the USSR (or its predecessor, Tsarist Russia), with a lot of tension existing between them over areas like Korea, Mongolia and Manchuria. These were all areas which underwent various degrees of jurisdictional flux in those years, as the Chinese empire crumbled. Japanese ambitions in mainland Asia led to the First Sino-Japanese War of 1895, whose outcome helped (along with Russia ambitions for a Pacific warm-water port) to bring about the Russo-Japanese War of 1905-1906. Japan’s intervention (along with other nations) in the Vladivistok region during the Russian Civil War, and its occupation of Russian territories in that area until around 1922 even after the other intervening nations had left, caused a lot of bad blood with Russia. It also fueled the ambitions of the members of the Japanese military who dreamed of acquiring large tracts of Siberian real estate. Also in the period from 1917 to 1922, Mongolia (on which Japan may have had an eye too) broke away from China with the help of the Russians and eventually became a nominally independent but heavily Soviet-leaning people’s republic. Japan’s 1933 invasion of Manchuria, which it turned into a puppet state, didn’t help to calm things down in that part of Asia either.
In other words, Mongolia and Manchuria were tinderboxes located on the overlapping outer fringes of three powers – China, Russia and Japan, all undergoing political convolutions of one sort or another – who for decades had been struggling with each other for influence in that area. To complicate matters, some of the borders involved were disputed and poorly defined. All of this finally led to a series of shooting incidents between Japan and the Soviet Union, starting in 1937 and eventually culminating in the Soviet–Japanese Border War of 1939. To quote what Hirohito would later say about WWII, the outcome of this border war with the USSR was “not necessarily favourable to Japan” – due in no small part to the fact that one of the commanders on the Soviet side was a talented fellow by the name of Georgi Zhukov, who clobbered the Japanese in the last two weeks of August 1939.
It’s at this point that events in Europe – specifically the signing of the Molotov-Ribbentrop Pact on August 23 – had a major ricochet effect on that little border war. The Japanese were furious that the Nazis, with whom Japan had partnered in the Anti-Comintern Pact of 1936, would reverse their position and become (at least on paper) buddy-buddies with the Commies, with whom the Japanese were at war at that very moment (and by whom they were being trounced). Japan therefore did essentially the same thing: it wound down the border war via a ceasefire and later (in April 1941) signed the Soviet-Japanese Neutrality Pact.
The Soviet-Japanese Neutrality Pact, however, didn’t meant that Japan had given up its ambitions on the eastern side of the USSR anymore than the Molotov-Ribbentrop Pact meant that Germany had given up its ambitions on the western side of the USSR. The Japanese Army remained convinced that the proper policy of Japan should be to expand “north and west” into China and Siberia. As I recall, some of its officers were completely serious about the concept of Japan taking control of all the Russian land east of Lake Baikal – which, as can be seen from a map, is not a trifling number of square miles. Overly ambitious as the Japanese Army plans may have been (particularly considering that it was doing badly in China and had done even worse against the Russians in 1939), the Soviets still took the threat of a Japanese invasion very seriously, despite the fact that it had a non-aggression pact with Japan. They took it so seriously, in fact, that it was only in the fall of 1941 – months after Germany has blitzed into the USSR – that Stalin transfered his elite Siberian troops from the Soviet far east (where the USSR was at peace with Japan) to the other side of the country (where the USSR was at war with Germany). If there was truly no chance of war between Russia and Japan in 1941, Stalin wouldn’t have left his best troops twiddling their mittened thumbs for months on the fringes of Mongolia and Manchuria while the Nazis were overrunning the European side of his country.
The reason why he finally moved his Siberian troops (in the nick of time to save Moscow, as it turned out) was that he’d received convincing information about Japan’s immediate plans from his top man in Tokyo, Richard Sorge, who – somewhat uncharacteristically – Stalin chose to believe. Sorge basically told Stalin that Japan, rather than following the Japanese Army’s wish to expand “north and west”, would be following that Japanese Navy’s preference for expanding “south and east” towards the D.E.I. (for oil), Malaysia (for assorted resources) and the islands of the central Pacific held by Britain and America (to extend Japan’s defensive perimeter). The Japanese Army wasn’t too happy about this: it still hankered to attack Russia, the officers of the Manchuria-based Kwantung Army being particularly inclined in that direction. What nevertheless won the case for the Navy’s “south and east” strategy (in addition to what should have been the self-evident fact that the D.E.I. were a much better source than Siberia for the resource Japan needed most, oil) was apparently the argument that the “south and east” strategy would be to a large extent a naval operation, and hence would only require a minimum number of Army troops. A huge percentage of the Japanese Army was still bogged down in China, where it had been fighting since 1937, so this limited the number of troops it could scrape together for any further campaign of expansion.
So in a sense, it could be argued that the chief factor which restrained Japan from attacking Russia in 1941 wasn’t the Soviet-Japanese Neutrality Pact, nor any lack of Japanese Army ambition or aggression directed towards Siberia, nor any lack of tension on the borders between the USSR, Mongolia and Japanese-controlled Manchuria. The restraining factor was actually the fact that Japan’s war in China was going badly. A war with the USSR could nevertheless still have broken out if Japan had shifted its priorities from China to Siberia, or if the Kwantung Army – which had a track record of provoking (on its own initiative) incidents leading to war – had taken matters into its own hands once again.