Montana's main deck was listed at about 7.05 or 7.35 inches thick. Air attack contributed to that protection, but the design also reflected an American preference for decisive long-range gunnery. British doctrine anticipated blinding enemy fire control at long range before closing toward roughly 18,000 yards, whereas American planning considered engagements at about twice that distance potentially decisive. At those ranges, shells arrived steeply and increasingly threatened horizontal protection rather than only the belt and hull sides. The heavy deck and turret protection therefore belonged to the ship's intended gunnery doctrine as well as its defence against aircraft.
Chapter · 33:02-36:39
Deck armour for plunging long-range fire
Thick deck armour answered air attack, but also the American expectation that decisive gunnery might arrive at ranges where shells plunged onto decks.