The Mogami class was designed from the outset so its triple 6-inch turrets could later be exchanged for twin 8-inch mounts. The same foresight could have extended to belt, deck, turret, and magazine protection. Japan was already understating the ships' displacement, so leaving them lightly protected meant accepting the cost of treaty cheating without securing the full combat benefit. Later bulges and added weight reduced speed, showing how difficult it is to restore balance after construction.
Chapter · 1:34:54-1:45:41
Mogami was prepared for bigger guns but not better protection
Mogami's planned conversion from fifteen 6-inch guns to ten 8-inch guns did not include an equally deliberate route to heavy-cruiser protection.