By the end of the war, as far as I know, Japan's navy was essentially obliterated. I'm wondering if the possibility of a blockade would have been feasible, assuming it was even considered.
And if it was considered - and obviously decided against - why? I feel like casualties on the American side could have been drastically reduced by not invading the home islands, and just 'laying siege' to Japan. Were the Americans simply war weary by then and wanted it over? Did the kamikazes play a role in deciding against it?
Thanks.
So a shorter answer tonight and then a longer one tmrw when I can crack some more sources open.
In a sense the USN HAD been blockading Japan since 1943 on. Submarines and aircraft had both completely destroyed her merchant marine and mined every major harbor, even operating with near impunity int he Sea of Japan itself.
But several considerations got in the way. One was the massive presence of Japanese forces still active on mainland Asia and in China who were still fighting and could be dangerous. There were also the Soviets, who were eager to get in on the spoils to be had in the Pacific and its not like the US could stop the Soviets from attacking the enemy if they wanted, and Soviet held land in mainland Japan was not ok with Washington.
Finally in a sense there were humanitarian concerns. While the bombings were deadly on a massive scale, both the conventional and nukes, they were predicated to force a quick end to the war and stop the killing as soon as possible, and so was the invasion of Japan. A siege was the exact opposite and only would have drawn out the killing, and would have weakened the military last, long after civilians had begun starving to death.
Hope that helps will come back with some more info about the blockade and its effectiveness in the morning. But by summer 1945 the islands were already essentially cut off from the world.
EDIT: the other poster hits the nail on the head. Top quality and adds many of the additional points I wasnt able to last night.
The book he recommend Downfall is a great read. I wish I had a copy and didnt have to rely on the library.
It was quite feasible - by that point in the war, Allied air and naval power utterly dominated the area around and above Japan. The country essentially was under blockade at that point.
Japanese shipping capacity had been destroyed for all intents and purposes, with about 700 ships (1,782,140 tons), mostly merchant shipping, sunk just in 1945 (Joint Army-Navy Assessment Committee report, February 1947, pages 79-99).
With that, Japan really had no reliable way to transport mass quantities of stuff (i.e. food, materials) by sea from the continent or among its own islands (which was how the majority of internal Japanese transport was conducted).
It's worth noting here that the Allies would not have simply just blockaded the Home Islands, but also continued to bomb conventionally. There was a targeting directive issued 11 August 1945 which ordered the redirection of bombing away from cities towards Japanese rail lines and transport infrastructure.
That, combined with an already-bad food situation in Japan, meant that any concerted blockade would've been most costly indeed for the Japanese in terms of starvation. In the event, things were quite dire; by 1945, the government was down to adulterating flour with things like acorns and mulberry leaves to stretch what little supply they had.
The "blockade & bomb" strategy does seem to have had its proponents, among them Admiral Chester Nimitz. He initially had supported an invasion, but went cold on the idea during the Battle of Okinawa, extrapolating the difficulty of subduing that island to the Home Islands of Japan. (Frank, Downfall, p. 147.)
On the other hand, the Army Chief of Staff - Gen. George Marshall - was committed to the idea of an invasion, in no small part because he believed that it was the only sure way to obtain a full capitulation by the Japanese and a lasting peace. He was also convinced by events in the European Theatre that air power alone wouldn't win the war in the Pacific.
As for blockade as an actual alternative to the Manhattan Project: It seems unlikely. By the time planners were seriously looking at options for concluding the war in the Pacific, the Manhattan Project had already been going for a couple of years, and was close to producing results.
NB: In chapter 21 of his book Downfall, Richard Frank examines the blockade idea as an alternative to the use of the atomic bombs or a full-on invasion, and explores the potential consequences of a blockade. Well worth a read.