The Japanese have the distinction of having the only two amphib operations in the Pacific to have been thrown back to the sea in abject failure. Wake Island and Bataan.
You should also add Milne Bay. There were several rather small affairs on Saipan and I think even Okinawa, all of which ended in failure. And for the allies there was the even bigger defeat at Dieppe and on the islands of Kos and one other that I forget now. The assaults on Iwo, Pellieu and Biak were all nasty affairs that by any objective analysis should be considered defeats. In Burma there were also several allied small scale amphib operation in the Arakan that were failures.
And lets not forget that the japanese undertook more amphibious operations with less troops, across a wider theatre of operations than any other nation in history
Now lets play fantasyland and say that 20 Japanese light divisions did get ashore on the west coast. There are only three worth while places to attack. Southern Cal, San Fran and Seattle. Which one are you going to take, because the others are several hundred miles away from each other. How are your forces going to operate over those distances that were in 1939, only really accessible by train?
Dont know, but i do know the Japanese were able to penetrate into the interior of China, with far worse terrain and far less infrastructure in 1937-38, with little difficulty. They were stopped in China by logistical difficulties and incresing levels of resistance by the Chinese, but only after having travelled approximately half the distance from the east coast to the west USA. I would say that the Japanese would have little diffulty in operating in the conditions that existed on the west coast USA in 1939, wher the levels of infrastructure were far more extensive, and the logistics far less demanding.
Now you say you are going to split your forces? For what? Get defeated in detail as the US industrial might gets mobilized?
Of course the Japanese have to split their forces If a Japanese Division is superior to a US Division in 1939, and the Americans have two divisions (unmobilized and not in position), and the Japanese are attacking with an assault force of say 6 Division, with another 14 following that, and they have the demonstrated and proven ability to operate in bad terrain and advance rapidly, what do you think they are going to do?
Light infantry you say? Well, the IJA was never known for being mechanized like the Allies and Germans were. For all practical purposes, there isnt enough industrial capacity in Japan to build the numbers of tanks (all of which were horrible designs) and trucks to do the task at hand.
Their tanks were not great, but in 1939 they were superior to anything available to the US Army. The best tank available to the US in 1939 was the M2, a predecessor to the Grant, minus the hull mounted gun. In the entire US there were the grand total of 18 of these tanks in existence in 1939, and there were no tanks actually in production until June 1940. To get even these grossly inferior vehicles off the production lines, with crews to man them and commanders to lead them would have taken more than a year to organize from the initial attack in December 1939.
Against this the Japanese had over 1500 tanks in total, all of which can be considered superior to the American Tank. These included the Type 89 (with a 57mm short barrelled gun, the Type 95 with a 37mm gun equal to the US machine in firepower. There were more than 100 Type 97s far superior to anything the US might hope to produce for at least two years). And there were hordes of tankettes and other AFVs all of which had proven useful in China, which at that time had formations better equipped than the US Army
Despite the low output of trucks that I agree with, the Japanese nevertheless managed to undertake major land based offensives, with very rapid rates of advance in Manchuria (in 1931), in China (1937-39, and again 1944-5), and also in Burma (1942). They wre the masters at improvisation and I do not see any great reason to suggest that this would not be repeated in this hypothetical...
You are going to march eastwards over trackless and waterless deserts through incredibly rough deserts. Then you are going to march through one of the great mountain ranges in the world only to find out you still need to go march several hundred miles through the great plains just to find out you need lots of boats to cross the Mississippi river. And all of this will happen without losses, or timetable upsets when the harsh realities of summer in the desert and winter in the mountains?
Err yes, I dont see the terrain in the US as being any more formidable than that faced by the Japanese in their advances in China in 1937-39, in Manchuria in 1931, in Burma in 1942, or perhaps most tellingly of all, across the Owen Stanley Mountains in 1942. The Japanese were not easily defeated by terrain, had the training the equipment and the organization to use their Infantry in such terrain, whilst carrying sufficient firepower at Battalion level to overpower most opposition they were likely to encounter in those conditions. In 1942, until their (ie the Japanese) supply ran out, AIF combat verterans had a hard time stopping them and this was done only after the Japanese, outnumbered and out of supply, had managed to advance to within 30 miles of Moresby. The Americans sent to fight them in the counteroffensive, were unable to cope with the Japanese at all, virtually falling apart as formations, despite having spent nearly a year in specialised jungle training. You might say the US was defeated by the terrain, but if, as you say the terrain on the west coast is that bad, I would think that would favour the Japanese rather than hinder them.
On the great plains, it would be more difficult, but again the japanese did demonstrate that they could handle even this type of warfare....remember your US formations are heavily outnumbered and lacking in mechanization in 1939, and the industrial capacity of the US is just not going to have much effect in the 3-6 months this campaign is occurring. It took the US economy more than two years to transform from a peacetime footing to a wartime economy, even with the massive injection of British Cash and carry funds to boost it along. In 1939 the US economy was sluggish and unresponsive, still recovering from the effects of the depression in many respects. I think the plains would be overcome by numbers....if we assume a month to overpower the coastal defences, occupy the ports and get them functioning, a month of fighting to cross the mountains, with say a loss of a division to both sides in this fighting, the Japanese are going to emerge from this battle with the equivalent of 29 divisions for the plains fighting to say 1.5 US regular Divs, say 3 National Guard units, and lets be generous say 2 Divs of these irregulars. As the Japanese this is a situation crying out for the standard Japanese Hook tactics, and as the American lines crack under this pressure, the rout begins with nowhere to hide....because we are on the plains remember......
And of course, the US industrial power will not do anything for months at a time and the Japanese army will be quickly supplied and reinforced by sea from bases 4000 miles away?
The Japanese were able to act more quickly than their opponents in 1941 when operating 2-3000 miles from their home bases, on mutiple fronts, and without the benefits of neutral shipping to support them. If the Japanese have neutral shipping to keep their domestic economy going, and can then concentrate their entire 6million tons of national shipping to the single front in this scenario, I actually think they would be better supplied than their scattered forces in 1941 could be. They demonstrated the speed that they could advance in China against stiffer oppositon than the US could put up in 1939, so why would I doubt their ability to advance quickly in this situation?????