Comrade, in this regard of the abilities of merchant vessels, it is worth remembering that worldwide, the speed of merchants was an average of 7.8 knots. (Estimate prepared for Tenth Fleet, which was responsible for the Atlantic seaboard, February, 1942).

ORIGINAL: Caput The obvious first question is why in the world would Japan want to invade the Pacific Northwest. Aside from natural beauty of course, a number of posters have already pointed out that the area had little to offer the Japanese from a resource or industrial standpoint. The cost of occupation is going to easily outweight any production you might see, especially when you factor in the poor local infrastructure, the unwilling populous, and the long shipping time back to the home country. I'd also say that outside of speculative fiction, its none to likely that Japan would then have the strength to march further east and conquer the rest of the US. Too large a disparity in industrial base, with the majority of US industry concentrated in the east, which is a heck of a long trip from Japan. So I think it would be safer to assume that Japanese conquest of the Pacific Northwest would be aimed either at achieving a peace settlement with favorable consessions (and sapping the US's will to fight), or perhaps as a coastal base of operations in a continuing naval war in the eastern pacific. Or finally, perhaps as a grander version of the invasion of the Aleutians. I think foreign soldiers occupying US cities would have had a galvanizing effect... in the context of WW2, this could provide the european axis powers a little breathing room, as the invasion of Europe is put on hold until after the liberation of Seattle. I think in the pre WW2 time frame, Japan would also need the help of another naval power, or at least US entanglement elsewhere. Even if you assume the Japanese achieve destruction of the entire US Pacific Fleet, they still need keep the remainer of the US Navy busy somehow. I agree with other posters, the logistics are daunting, and the sealanes between Japan and Seattle need to be completely secure, and that will be hard to accomplish if the Atlantic Fleet is still available (or the British Asiatic Fleet.. they can't be too keen on having the area adjacent to British Columbia invaded). So I think we're basically at the point of needing to invent an 2nd 1920's war to create enough turmoil that our invasion is possible. So rather than speculate an entirely new war, lets stick to the one we have, WW2. Assume the Japanese victory at Pearl Harbor is total. The battleship fleet AND the carriers are all out of action. This is quickly followed up by an invasion of Midway and Hawaii, which happens to also capture a large portion of the US submarine fleet. I think the obvious next move have been to consolidate things throughout Asia, perhaps invading Australia as well. Get that 'Greater East Asia Co-Prosperity Sphere' thing really going. But lets say the Japanese don't see it that way. They know the US is down, but not out, and they want the US totally knocked out of the pacific war. If humiliation in Hawaii wouldn't do it, perhaps bombers over Hollywood would? (G4M Betty could reach LA or Denver from Seattle/Portland). A little research on Wiki shows that Japan had a prewar merchant fleet of about 6,800,000 tons. Assume most of the fleet is needed in the Pacific moving raw materials, but 10% could be pulled to supply an invasion garrison, giving us 680,000 tons. Tokyo to Seattle is about 8000 nautical miles round trip, or about 1000 hrs @ 8kts, or about 42 days. Lets call it two months round trip, due loading/unloading and forming up convoys (while we're at it, lets assume that Japanese ASW is competent). So we can move about 340,000 tons/month. Some quick googling shows that a combat division in ww2 required anywhere from 100-600 tons/day. I think we can safely pick 200 tons/day, since after the initial invasion I think our Japanese army will be hunkered down in the favorable mountain terrain. In addition, some of the foodstuffs can be gotten locally, possibly from Japanese fishing fleets now based out of Seattle. So 6000 tons/month for a division. With that kind of consumption, the Japanese could comfortably maintain over 40 divisions, though 25 might be a more realistic number, since some supply will go towards airpower and naval replenishment. 25 still isn't too bad, given that the Japanese army strength in 1941 was about 41 division (27 of which were committed to China/Manchuria). So, I'm surprised, but I think if Japan could establish secure lines of communication, manpower limitations would be more important than logistics. But even 10 divisions would be pretty formidable against a reeling 1942 US army (26 divisions). Anyway, it's fun speculation. My calculations are all back of the envelope type stuff, if anyone sees any errors or grossly incorrect assumptions, feel free to call me on it.

There is a lot of oil in California.

One interesting possibility does raise its head though. If the Japanese had invaded and occupied a good chunk of the US (say everything West of the Sierra-Nevadas), how likely is it that a serious source of resistance might have emerged from the ranks of the American Communist party? How might that have changed the postwar political landscape in the US? The Communists were never major political players in the US, but under the right circumstances (Lenin in WW I or Tito and Mao in WW II, for example), the Communists showed themselves to have a great propensity at organizing armed resistance and political consensus.

* By mid-1942, the Japanese Navy--both the warships and the cargo ships--needed a rest. They had been operating at an unsustainable, even a breakneck pace for half a year. Ships can't sail 7 x 24 all year long, and much of their fleet sorely needed overhaul by mid-42. That let-down in Japanese naval abilities had something to do with their inability to push a small beachhead of Marines out of the Solomon Islands for something on the order of six months.

* Most of this discussion has neglected the possible role of the US Air Force. While B-17s didn't have much success bombing mobile naval targets, they had a much better track record against ships in harbor, trying to load or unload cargo. While the Japanese air force did a good job in the early days of the war, they would have been relying on carrier-based planes to provide aerial supremacy in the early weeks of the war. Given the challenges in keeping carriers on station 2,500 miles from their home ports (5,000 if they're sailing from Tokyo rather than Hawaii), this seems significant.

* The Japanese were never even able to mount a successful invasion of Australia, which presents only a fraction of the logistical challenge that an expedition against the US would have. And they would have been fighting a territory with a population and industry smaller than the state of California.

Their transports, however, would not have been in the same state of readiness. What none of us here have any hard data on is "just how busy Japanese transports and cargo ships were maintaining the status quo".

After taking a look at the map of the western US (and noting that capturing Alaska wouldn't help the Japanese), I'd say a landing on the "peninsula" west of Seattle, north of the line Olympia-Aberdeen and perhaps south of that line would be possible.

ORIGINAL: Seawolf I don't think political will was so weak then. It became so only after the mass-treason movement of the 60's and the vindication of the seditious media by Walter Cronkite and others. The resistance would have been different in nature than that of China because China already had its disunity problems. The question is how the Japanese would have succeeded in the west-coast terrain.

ORIGINAL: Seawolf I don't think political will was so weak then.  It became so only after the mass-treason movement of the 60's and the vindication of the seditious media by Walter Cronkite and others. 

In the long term, even if they had satisfied all those conditions, I don't think the outcome of WW II would have been so different, with the exception of taking an extra 2-3 years and involving many more civilian casualties in Japan.

I believe the whole starting point for this discussion was that the attack on Pearl Harbour also knocked out the carriers in the Pacific. That would've meant the US would not be able to mount any form of offensive until around late 1942. In this scenario, a "Midway" is also unlikely, and all Japanese carriers would probably still be in operation.