An interesting question arises. Could the Japanese have effectively taken our west coast? I submit only if the following had happened.
1. Germany invades our east coast, or otherwise occupies our Atlantic fleet.
This would have been highly unlikely. Germany had no aircraft carriers (the Graf Zeppelin was never completed), limited surface ships (most of which had been bottled up near Germany or in France after the Bismarck debacle), no amphibious warfare capability, and no long range aircraft capable of reaching the U.S. The logistical problems alone would have made it virtually impossible given that the Royal Navy was one of the most, if not the most powerful in the world at the time.
Also, by this time, the British were routinely reading the German naval codes via ULTRA, making any German invasion very difficult to disguise or hide.
2. Germany stages from Mexico.
Germany actually had an interest in getting Mexico either on their side, or getting it to agree to German troops there. It never happened, but again, the logistical requirements alone would have made this extremely difficult. Given that the Germans couldn't sustain a campaign in Europe (see Russia), their ability to do so across an ocean hardly inspires confidence.
3. Russians side with the Japanese and has the logistical capability to support over the horizon expeditionary operations from mainland Russia/captured American Pacific Islands.
The Russians would have had the same problems the Germans would have had. In 1941, the Soviet Navy could have charitably been described as modest in comparison to the U.S. Navy and the Royal Navy. It's composition in 1941 was:
3 line battleships
7 cruisers (including 4 modern Kirov-class cruisers)
59 destroyer-leaders and squadron-destroyers (including 46 modern Type 7 destroyers and Type 7U destroyers)
218 submarines
269 torpedo boats
22 patrol vessels
88 minesweepers
77 submarine hunters
and a range of other smaller vessels
In various stages of completion were another 219 vessels including 3 battleships, 2 heavy and 7 light cruisers, 45 destroyers, and 91 submarines.
By this time, the Soviets were decisively engaged against the Germans, and many of their ships were in ports defending against German air and naval attacks. Even had they sided with Japan, it's unlikely they would have released any of their ships given the threats they faced against the Germans. Also, any attack on the western US would have entailed transporting large numbers of troops thousands of miles, and logistically sustaining them. If one looks at what the Allies had to do in France, Italy, and North Africa, one gets an idea of the enormity of such an endeavour (we built "Mulberry Harbors" out of concrete that were towed across the English Channel for this very purpose, an astonishing feat of military engineering). Given that the Soviets did not have a Navy capable of such actions at the time, it is highly unlikely they could have contributed significantly to any attack on the US west coast.
4. Japan finds our carriers and sinks them.
The loss of our carriers, while catastrophic, still would not ensure the Japanese of a successful invasion. As noted above, the logistical requirements would be huge, and something the Japanese would have been hard pressed to sustain given the distances and the forces they had available at the time. They'd have to take Wake Island (which they did), Midway (which they did not) and Hawaii (which they did not) in order to make this scenario even remotely possible. Even then, it would have been very difficult for them. They also knew at Midway that their chance to end the war on their terms ended when the Kaga, Akagi, Soryu and Hiryu (Japanese fleet carriers) were sunk (we lost the Yorktown). My take is that as much as they may have wanted to, the Japanese knew that an invasion of our west coast was not only improbable, but highly unlikely.
Of course, none of this happened. Our west coast may have been "invasion ready" as you note, but for the reasons I posit above, an invasion by the Japanese would, I submit, have been doomed to failure.
