I’ve been doing some thinking about potential Japanese invasion scenarios. If you’re invading a small island, you can load some soldiers up onto transports, travel a very long distance, and know that when you arrive your combined land/air/sea force should be strong enough to overcome the forces on the island. But invading a mainland is different. The number of soldiers you can fit on your transports is likely to be much smaller than the number of enemy soldiers on the mainland. You don’t really have the option of dropping off one batch of your soldiers, sailing a long distance to your homeland, picking up another batch, and using the second batch to support the first. If you tried that, your first batch would be wiped out before the second could arrive to support it.
To solve this problem, you need a base for the invasion near the place you’re going to invade. That way your transports can make a lot of back-and-forth trips quickly so that your invasion force will grow large enough to resist the enemy’s mainland force.
The most logical invasion route for the Japanese to have used would have consisted of the Aleutian Islands and Alaska. During the real war, Japan took one or two of the Aleutian Islands. The Aleutian Islands are position in a nice, long chain–a chain which seems to reach out from Alaska toward Russia and northern Japan.
Suppose that on December 7th - 8th of 1941, Japan had followed up its first two air attacks against Pearl Harbor with a third air attack. (As a good naval commander would have done.) Also under this scenario, Japan would have used its powerful surface fleet to have bombarded Pearl Harbor’s facilities. Then it could have sank the heart of the U.S. carrier fleet in the scenario Clyde described. These activities would have been intended to render both the U.S. Pacific fleet and the naval base at Pearl as impotent as possible, as quickly as possible.
After these tasks were finished, a large portion of the Imperial Japanese Navy would have headed north, to begin an island hopping campaign through the Aleutians. December or January may not seem like a great time of year to begin an invasion of Alaska. However, coastal Alaskan weather tends to be considerably milder than inland Alaskan weather. Coastal Alaskan weather is like Seattle’s weather, except with colder rain.
This island hopping scenario was well within Japan’s capabilities, especially under the favorable naval scenario outlined by Clyde. Within a few months of the Pearl Harbor attack, Japan could have taken possession of whichever of the Aleutian Islands are closest to the Alaskan mainland. That island could have served as a base for the invasion force.
At this point, it could be pointed out that Japan’s ability to transport a large invading force across a body of water in 1941 was considerably smaller than the U.S. and Britain would have for the D-Day invasion of 1944. In addition, it would not surprise me if the Aleutian island in question was farther away from the mainland than Britain is from Normandy. However, there are several factors which would offset this. As Clyde has pointed out, the U.S. Army was not very well-prepared for war in early '42. Witness, for example, the early embarrassments it experienced in its Algerian invasion. Had General Patton not subsequently been given command, that situation could have spiraled out of control.
Another offsetting factor was that the portion of the Alaskan coast vulnerable to invasion was much, much larger than the Normandy/Calais area Germany had to defend. Also, Japan had a very strong surface fleet, including the most powerful battleships ever built. American land forces could be bombarded along a wide stretch of Alaskan coast, and could also be subjected to air attacks from carrier-based planes. Given enough time, the Japanese could also construct airfields on the Aleutian islands they’d captured.
All of this might–might!–be enough to allow Japan to land an invading force in mainland Alaska, and not have that force get instantly pushed back into the sea. In late '41, Japan’s planes and pilots were often better than their American counterparts; giving Japan a very fleeting air superiority. I say fleeting because the U.S. very quickly improved its aircraft designs, and used its overwhelming industrial advantage to heavily outproduce Japan. When Japan attacked Pearl Harbor, the U.S. had ten times the industrial capacity of Japan! During WWII, Japan gradually reduced that margin of superiority by industrializing itself. Nevertheless, in any long war between the U.S. and Japan, there could be only one outcome.
Presumably, the objective of the Japanese Army would be to move southward along the Pacific coast until it had reached the Mexican border. The Imperial Japanese Navy could intervene as necessary, delivering reinforcements and supplies where needed, while also shelling American positions. But it is a very long way from Alaska to Mexico, and the Japanese Army would have to contend with its American and Canadian counterparts along the way. A conquest like this would take time, even if it could be accomplished at all. (Which is highly doubtful.) Even if the invading force somehow reached Mexico, the heart of America’s war making and industrial capacity would remain intact. The next phase would involve a massive push eastward. The West Coast would be the starting line for the push, and the Atlantic would be the finish line. However, it is highly doubtful that the Japanese Army had the strength for such a massive undertaking.
Since we’re painting a best-case scenario for Japan anyway, I’ll borrow an idea from Herr Kaleun. Suppose that in the wake of the U.S. oil embargo, the Japanese ambassador to Mexico had begun achieving brilliant successes. Suppose he’d persuaded the Mexican government to begin arming itself in preparation for a future hypothetical war against the U.S. Presumably, Mexico would initially sit on the sidelines. But the more successes Japan’s Alaskan invasion achieved, the more likely Mexico would (presumably) be to enter the war. The Japanese ambassador could instill a sense of urgency with comments like, “the sooner you enter the war, the greater your rewards will be.”
My sense is that if these things were to have occurred, the Japanese Army would have been stopped partway through Canada via a joint U.S./Canadian effort. Mexico’s invasion would likely have fizzled out after a few relatively small initial victories. Things would be stagnant for a while, after which the U.S. would use its overwhelming advantage in industrial capacity to push back. The main effect of the Alaskan invasion would be that the U.S. would be sidetracked from its Kill Germany First strategy in order to deal with all this. That distraction element would probably have postponed Allied victory in Europe by a few months, or perhaps even a year.