>> Does "vital" in this mean Op Downfall cannot be supported at all, or mean extensive and expensive alteration of preparations, or something else I am missing?
Short form Yes, the Philippines were vital. Without Formosa, somewhere had to be the logistical node between the West Coast of the USA and Japan to stage combat forces to invade the Japanese home Islands
See these definitions from "History of Planning Division, ASF. Volume 5 -- 1799"
Staged Division Manpower slice -- 25,000
Assault Division Manpower slice - 40,000
Directly quoted passages defining ASF project for Philippines base development in another volume of History of Planning Division, ASF --
1. "Logistic Study for Projected Operations, ASF-P-SL-5,11 informally furnished to technical service representatives on 23 February 1945, is one of a series on operations in the Pacific and deals with the maximum base development of the Philippines. This study will serve as a basis for preparation of War Department projects and for development of troop and other personnel requirements for service installations.
and projects to provide emergency resupply on Luzon for Olympic and Coronet.
d. Emergency supply for KYUSHU and HONSHU operations.
After forces have been mounted in the PHILIPPINES for the
operations against KYUSHU and HONSHU, supply will be from
the UNITED STATES with emergency supply only from LUZON.
On LUZON, a 30-day emergency stock of all classes of supplies will be maintained as follows:
For 13 divisions at 40,000 men from D+10 to D+90 of
KYUSHU operation (see ASF-P-SL-2). Supplies available for
For 25 divisions at 40,000 men from D+0 to D+150 of
HONSHU operation (see ASF-P-SL-1). Supplies available for
HONSHU operation. Second priority KYUSHU operation.
When you go look up "Project ASF-P-SL-1" in "History of Planning Division, ASF" you find in addition to coronet, there were three to four further nine division operations to complete the conquest of the Japanese Home Islands in the Summer and Fall 1946 plus a 25 division expeditionary force to China in the Fall-Winter 1946 to complete the extermination of the Japanese armies in China.
These planned for operations are where General Marshall et all talk about 1,000,000 American casualties from a conquest of Japan.
Assuming four each nine Division assaults -- Hokkaido, Northern and Southern Honshu, plus Shokaku -- That is 27 divisions at 35% casualties...Plus 25-35 more Divisions in China operations...
35% of a 20K combat element division slice is 7,000 casualties.
7,000 times 62 divisional assaults = 434,000 casualties.
All AFTER Coronet.
I'd say General Marshall's figures were well documented. And his "Atomic Diplomacy" academic critics are idiots.