Up to the end of the First World War, cruisers served as scouts for the main battle fleet, as commerce raiders or escorts, and to show the flag in distant colonies. For these roles, 6” guns were perfectly adequate, and this was the standard cruiser armament. Cruisers were somewhat larger than destroyers, were generally lightly armored, and were fast, with a speed of 30 knots or better.
However, the disarmament treaties limited non-capital ship gun caliber to 8”, and suddenly this became a new standard for cruiser armament. Ships whose guns met the older standard of 6” caliber became known as light cruisers, and those with the newer 8” standard caliber became known as heavy cruisers.
With the lapse of the naval treaties and their limits on displacement, cruisers began to put on more armor protection, particularly in the U.S. Navy, whose cruisers began to resemble pocket battleships. Japanese heavy cruisers also had relatively heavy armor protection.
The Pacific War Online Encyclopedia (c) 2007 by Kent G. Budge. Index