Why didn’t Japan attack the West Coast of the United States during World War II?

We must never forget that Germany and Japan, the Axis powers, were also developing nuclear weapons in WW2, but before the Axis could finish development and deploy their nuclear weapons or dirty bombs, the Allies won the war.

The German submarine U-234 was delivering nuclear materials from Germany to Japan when Germany surrendered. That German submarine was ordered to surrender by the German high command and the two Japanese officer passengers on that submarine committed suicide. https://en.wikipedia.org/wiki/German_submarine_U-234

The Japanese planned to use these nuclear materials to make dirty bombs since they had not yet been successful in completing an atomic bomb. The Japanese had already dropped biological weapons on China and planned to do the the same to the American West Coast. https://en.wikipedia.org/wiki/Operation_Cherry_Blossoms_at_Night

We rarely hear about Germany and Japans nuclear development efforts since they did not finish them in time to use them. Some say the Axis lost the nuclear arms race in WW2 because Germany discounted the value of theoretical physics over the applied sciences, thus allowing Jewish scientists to study theoretical physics before fleeing persecution in Germany leaving a knowledge gap while adding considerably to the Allies knowledge of in this area.

Another bombing of the Continental US by Japan that was not mentioned yet was the Japanese bombing of Oregon via submarine-launched bomber planes.

The Lookout Air Raids are bombings that were carried out by the Japanese Navy against the American mainland by using incendiary bombs to start forest fires and therefore divert American resources to fighting fires.

However, the bombs failed to cause a fire, rendering the operation more or less useless.

An interesting footnote to this story is that one of the pilots involved, Nobuo Fujita, returned to the town he bombed after the war to express his remorse, and was favourably received during his visit by the citizens of Brookings, Oregon.

Another attack I don't see mentioned here yet was on the SS Montebello off the Central California coast near Cambria. From Wikipedia:

SS Montebello was an oil tanker sunk by the Japanese submarine, I-21, off the coast of California on December 23, 1941.

https://en.wikipedia.org/wiki/SS_Montebello

Why didn’t the Japanese attempt even a single attack on the mainland? What deterred them from striking such a direct blow on America? A submarine attack would have been easy for them, but they never sent a known submarine beyond the territorial waters of Hawaii.

In addition to the other answers, I'll mention the I-400 class submarine. They were under water aircraft carriers and each carried 3 M6A seaplanes. They were conceived in 1942 as a way to bring more than just a submarine's one little deck gun to the US west coast.

The Japanese knew sending a surface fleet to raid the US west coast would be a nightmare. Going beyond Hawaii meant an extra 5000 miles round trip. 5000 miles of extra fuel tankers and logistics ships. 5000 miles of patrolling and very angry US aircraft, ships, and submarines. Even if successful those ships would be out of action for weeks while in transit. Their solution was the I-400.

The I-400 class married the stealth and operational range of a submarine with the strike range of small bombers. Instead of a normal submarine firing at the coastline with its little deck gun, the I-400 could launch three bombers which could head inland and attack vital industrial and transportation targets.

Unlike many Axis "super-weapons" this one worked! But also like many Axis super-weapons, it took too long to reach the battlefield and there weren't enough. 18 were planned, 3 were completed, only 2 entered service before the war ended.

The plan was to attack the Panama Canal and stem the tide of US ships flooding from the East Coast to the West. The Panama Canal was well guarded against conventional surface, air, or submarine attack. The thinking was that a fleet of I-400s could sneak up and launch its aircraft late at night. They could do this in 30 minutes. The aircraft would attack from an unexpected direction bypassing the defenses and having the element of surprise.

By the time they were completed the war was basically over. And like everything else in the Japanese arsenal it was planned to be sent on a suicide mission against the US fleet. But the elaborate plan failed and the submarines were captured at the end of the war.

There's a good documentary on the I-400 by World War II in HD Colour.

I've been in excavation for 30 years...all that to say...I love "Man and Machine" and I have paid close attention to some of the logistical side of man-made machines.

I believe the "hit and run" sub attacks created good psychological fear along with the balloon bombs. But to amass a fleet of ships to actually sail the distance to American coast lines would have been a suicide mission. They would be too far from "support" to carry out a continued attack and land invasion.

My understanding is, we, at the time had a very effective civil air patrol very willing to patrol our skies...and they did. This would have helped a great deal in spotting ships afar.

The following map - somewhat ironically - was recently granted the prestigious Grand Award of Japan for design. As you can see, the distance between Japan and the United States is quite large.

Another geographic feature that simply dealt a better hand to the Americans in WWII was island placement. Hawaii, being somewhat halfway in between the U.S. and Japan is pretty much your last stop till you hit California. That's a long supply chain to deal with, even if the Japanese decisively conquered Hawaii. And even then, if the Japanese successfully invaded the West Coast and took over major cities like San Francisco or Los Angeles, they would have to deal with the sprawling network of railways and automobiles which could bring Americans to the front lines much faster than a Japanese resupply coming in from Hawaii, much less Japan itself.

It is also worth noting that there is a decently spaced chain of islands and atolls that stretch from Hawaii to the Philippines and southern Japanese islands like Okinawa, which put Japan on the defensive for much of their fight against America. After the Japanese conquered America's Pacific outposts, it had to hold them at all costs to stave off what eventually became their inevitable defeat.

Japan, like all major navies, had doctrines that were highly influenced by Mahon's book The Influence of Sea Power Upon History published in 1890, which emphasized the critical role of capital ships (battleships) in defensive roles in the home waters. The US Pacific Fleet was deployed forward, at Pearl Harbor, in an position to conduct offensive operations throughout the Pacific. Japan could not execute its strategic plan of dominating China and Indonesia with the US threatening its rear.

Ironically, the very success of the attack on Pearl Harbor rendered the battleship obsolete except as a floating artillery battery against land targets. A single pilot, flying an aircraft that could be quickly an cheaply produced, dropping an inexpensive bomb or torpedo could cripple or sink a very long leadtime, expensive battleship with thousands of crew.

Yamamoto immediately recognized the significance of his accomplishment at Pearl and set out an elaborate plan to draw the few US aircraft carriers into battle near Midway. Part of his plan was the diversion in the Aleutian Islands with the occupation of Siska, for domestic propaganda purposes, and an attack toward Dutch Harbor, which is the only deep water port on the Great Circle (i.e., shortest) route from Japan to the US. Thanks to breaking the Japanese naval code and a lot of skull sweat, Nimitz knew that would not be the main event. The battle of Midway, just months after Pearl, was a stunning defeat for the Japanese Navy, which lost 4 carriers in 5 minutes. After that, the US had the strategic initiative and rolled up the enemy outposts along the chain of islands leading east to New Guinea and then north to Okinawa and eventually the home islands.

From June 1942, Japan was on the strategic defense. There was little coastwise commerce along the West Coast to disrupt (mainly timber, which could be moved by rail, if necessary). There was a high probability of surface craft being detected and attacked by land based aircraft, and the flow of materiel from San Francisco, Seattle, Los Angeles and San Diego was convoyed and at the outer limits of submarine operations. Japan lacked a strategic air arm capable of inflicting substantial damages and casualties on American cities. There was no likelihood of inflicting sufficient damage to bring the US to negotiations.

In short, offensive operations against the US mainland were high-cost, low-payoff missions that detracted from the fierce rearguard retreat toward the home islands.

Range of their ships...

The Japanese doctrine dictated a naval battle near the home islands, a repeat of the Russo-Japanese war. They did so largely because they lacked enough fuel to reliably reach, much less maintain any kind of battle fleet off the coast of the U.S.

The major reason that a Japanese carrier attack on Pearl Harbor was of low probability was that it was believed the Japanese lacked the ability to refuel underway, especially in the North Pacific in the winter. Such refueling was absolutely necessary because a battle fleet absolutely required the protection of its destroyers and destroyers were small fast ships that carried little fuel and quickly burned through that which they did.

The U.S. had only managed to accomplish underway refueling under simulated war conditions in late 1939 using a newly invented and highly secret disposable fast decoupler for the fuel lines. Knowing the Japanese lacked this technology, it was thought unlikely they could reach Hawaii.

But as usual, the Japanese used hyper-intensive training and much higher tolerance for casualties to build the skills necessary to perform such refueling. Most of those superbly trained men died at Midway trapped in the inferno the hangar decks of the stricken Japanese carriers turned into, after the American bomb strikes.

Fuel oil ruled the Pacific War in 1942. The U.S. stupidly let itself get by with only three fleet oilers (tankers who could keep pace with battle fleets which traveled at twice the speed of regular merchant ships and tankers). Two were sunk or damaged and the U.S. relied on civilian tankers to support all their operations. See: Black Shoe Carrier Admiral: Frank Jack Fletcher at Coral Sea, Midway & Guadalcanal.

Every one of the battleships sunk and then razed at Pearl Harbor were relegated to patrolling the West Coast because they did not have either the speed or range to keep pace with carrier task forces. Neither did the U.S. have enough tanker capacity to support them away from home ports.

The real question is why the Japanese did not utilize their submarines like the Germans. Had they deployed submarines along the West Coast as the Germans did during Operation Drumbeat, they could have devastated the U.S. pacific merchant fleet and crippled all offensive actions.

They didn't do so because they were fighting a scripted war. They knew they could not defeat the Americans materially in a long war, so they all deluded themselves that the war would be decided by a quick devastating destruction of the Pacific fleet in one massive decisive battle. At which point the Americans who where nothing but greedy, craven businessmen, would beg for peace.

They originally planned on luring the U.S. Fleet to a decisive battle somewhere between the Philippines and Okinawa. The sole doctrine for the Japanese subs was to whittle down the U.S. battle fleet as it traveled across the Pacific to the area of the decisive battle. Sub commanders were ordered to ignore merchant vessels and only attack warships.

Worse, the doctrine required large fleet subs, able to travel on the surface with the main battle fleet. This made the sub too large to dive quickly, so they suffered high losses when they ventured too far into areas where the U.S. had dense air patrols. Even if they had decided to deploy their ship in anti-logistics attacks on the west coast, they didn't have subs suitable for such a mission.

The Japanese inflexibility to the point of delusion, gave the U.S. a year to build a real logistics system and unlearn all the mistaken doctrines the Navy had developed after not having fought a serious naval war since 1898.

In the end, the Military, who thought of themselves as Samurai whose only role was war, were so desperate to maintain their role and status that they deluded themselves they could win a war with U.S. But they could only win a war that followed a specific narrow script. The script said that the war would end long before logistics issues came into play; they did not plan on attacking U.S. logistics nor did they rush to develop the Indonesian oil fields.

Japanese submarines did come to the American Pacific coast. I know of one bombardment while visiting Fort Stevens on vacation. The attack did no damage, but appears to have helped foster fears of further attacks.

In the strategic sense, basically nothing happened on the American Pacific coast to change the course of the war, but one could argue that the defenses were a huge diversion of resources to guard against a threat that in hindsight was fairly marginal. However, had Japan won the Battle of Midway, then strong Pacific coast defenses would have been an even higher priority.

Japan had a lot of targets closer to home. They included Southeast Asia (which they conquered), China, and India (Japan came fairly close). They did attack American possessions on the far side of the Pacific such as Wake Island, Guam and the Philippines, plus Midway and the Aleutians, but otherwise, pretty much left mainland America alone after Pearl Harbor. Only when Japan had run out of Asian targets to attack, when the choice was between Australia, Iran, and San Francisco, would it make sense to attack the U.S. west coast. By that time, the Japanese planned to have a behemouth consisting of Japan, China, India, today's "ASEAN" nations, and maybe Siberian Russia behind her, but not before.

The Pacific was a relatively low priority theater for America, because it took America twice as many supplies to maintain one soldier against Japan in Asia versus Germany in Europe. Likewise, it would take a multiple more supplies for Japan to sustain an attack on the U.S. west coast versus say, China.