[Matt Wiser]

The plan for gas in DOWNFALL was not finalized, but Hap Arnold was on record as being against strategic use, as the industrial targets meant to be hit by gas would've been burned out by B-29s anyway. Limited, tactical use of gas was far more likely, and troops going ashore would've had masks, protective suits and gloves, and four Liberty ships with gas munitions were to be offshore on X-Day in Kyushu (scheduled for 1 Nov 45). After Iwo and Okinawa, the plan was to use gas on those cave and bunker defenses that had resisted all other efforts to subdue (i.e. the "blowtorch and screwdriver" method of flame throwers, explosive charges, and direct fire from tanks and artillery), that were in the line of advance. The agents used were likely to have been Phosgene and Cyanogen Chloride. And in tests at Dugway in Utah, those agents were tested against mockup cave and bunker defenses, with gas concentrations five to ten times as dense inside the cave as one got outside, enough to penetrate gas mask filters and kill masked Japanese. All that one needed was formal Presidential Approval, but that had not been given prior to the Japanese surrender.