Did the Japanese attack the US West Coast during WWII?

(My answer on Quora)

As others have mentioned the Japanese did indeed attack the West Coast of the United States. The most newsworthy of these was the naval bombardment of Ellwood in Santa Barbara, California that damaged a local oil refinery. This occurred on the 23rd of February 1942.

The military base of Fort Stevens in Oregon was also shelled by the Japanese Submarine I-25 on June 21st 1942 with very minimal damage. In addition the Japanese occupied two of the Aleutian Islands (Kiska and Attu) prior to the Battle of Midway with the intention of protecting their Northern Flank. The Attu landing marked the first time a part of the continental United States had been occupied since the War of 1812.

So why then did the Japanese not go any further? Well the defeat of Kidō Butai (1st Fleet) at Midway put an end to further aspirations on the West Coast by securing naval supremacy in the Pacific in American favour.

However lets place devil’s advocate for a moment and assume that the Imperial Navy had emerged victorious on the 7th of June 1942 with Admirals Yamamoto and Kondō defeating Nimitz, Spruance and Fletcher.

A victory here would have greatly turned the tide in the Pacific in Japan’s favour. It would only have been a matter of time before Pearl Harbor fell and with it Hawaii. With its new position the Japanese could have launched further sporadic attacks against the American West Coast and probably would have to limit an American Naval rebuild. It is very unlikely that the US would have sued for peace even in this alternative scenario (not with the home front intact).

As for a full scale frontal attack on the West Coast proportions this is improbable. The planning and logistic commitments here would have dwarfed D-Day. The expanse of ocean that would have to be traversed was far too wide and the Japanese for all their maritime success did not have the Armed force strength of the Wehrmacht. Their continuing struggles in China and their defeat at the hands of the Soviets/Mongolians at Khalkin Gol had made that clear already. All of this likely played a significant part in the decision of Japan to not open a Second front in the Soviet Union much to the annoyance and ongoing frustration of Adolf Hitler. Invading the US mainland was even worse of an option. This is not the same as an assault against American positions in the Philippines. Far from it.

Besides which Japan was already heavily committed elsewhere having greatly expanded her domain in the Malaya Peninsula (including Singapore), Hong Kong, the Dutch East Indies, the Philippines, French Indochina, Burma and numerous Pacific Islands. Her intentions for the time being would likely have involved the consolidation of gains (against active resistance) with an eye to containing future American rebuilds via a naval check. Anything beyond this would have spelled disaster.