1920s: Washington Treaty
The Washington Naval Treaty of 1922 imposed a moratorium on new battleship construction, with the exception of the two Nelson class battleships by Great Britain, and set very strict limits on the tonnage and firepower of future battleships and battlecruisers. It also set the definition of a capital ship as a warship of more than 10,000 tons standard displacement or with armament of a calibre greater than eight inches (203 mm). There was the concern that a subsequent race in building larger, more powerful cruisers might subvert the usefulness of the prohibition on capital ship construction and encourage navies to squander their now-limited perimissible tonnage for capital ships on fast vessels designed specifically to hunt down large cruisers. To avert these challenges, representatives of the United States, Great Britain, Japan, France and Italy set limits on the tonnage and firepower of cruisers to 10,000 tons in displacement 8 inches (200 mm) for maximum main gun caliber. These limits were in the interests of the U.S. and Britain especially. Planners in the U.S. Navy had spent two years prior to the start of negotiations designing 10,000 ton, 8-inch cruisers and were convinced that smaller vessels would not be worthwhile. Britain had just built its Hawkins class cruisers and wanted to ensure they would not fall prey to a much larger type of super-cruiser.
Despite these intentions and set limitations, a number of new, powerful cruiser classes emerged from these nations, which sparked off something of a cruiser arms-race. The Japanese navy had a doctrine of building more powerful ships in every class than its likely opponents, which led to the development of several very impressive heavy cruiser classes. British and American building was more influenced by the desire to be able to match the Japanese ships while keeping enough cruisers for their other global responsibilities. With battleships heavily regulated by the Washington Treaty, and aircraft carriers not yet mature, the cruiser question became the focus of naval affairs. The British, with a strained economy and global commitments, favoured unlimited cruiser tonnage but strict limits on the individual ships. The Americans favoured the opposite: strictly limited numbers of powerful cruisers. Disagreements between the British and Americans wrecked the 1927 conference on naval affairs.
Even during the 1920s, the 10,000-ton limit was not always strictly observed, although British, French and American designers generally worked to the limit with precision. The British built 13 of their famous County Class with four twin 8" gun turrets but with very minimal armour. The ships had fine sea-keeping qualities and a long range, but were virtually unprotected, and were easily damaged in combat. The Japanese Myoko class, however, grew during its construction as the naval general staff prevailed on the designers to increase the weapons load. As well as a breach of the Treaty, this was a poor decision from the design point of view and the ships had to be reconstructed in the 1930s to reduce weight. The German Deutschland class, was classified as armoured coast defence ships under the terms of the Treaty of Versailles. They superficially resembled contemporary battleships due to their massive main gun turrets and unusually high conning tower/bridge. However, they were in effect a heavy cruiser being upgunned to 11-inch batteries at the cost of slower speed; their displacement was declared at 10,000 tons but was in practice considerably greater.
The Pensacola class cruisers were the US Navy's first "treaty cruisers" designed in line with Washington Naval Treaty restrictions. Their main battery consisted of ten 8 in (200 mm) guns, in two twin turrets on the main deck, and two triple turrets two decks above, making it one of the two US Navy ship classes (besides the Nevada-class battleships) to have different-sized turrets for main armament. Their thin armor on the belt (varying from 2.5 to 4 inches (63 to 100 mm) in thickness) and deck 1.75 inches (44 mm) was no better than that on 6-inch-gunned cruisers and was inadequate to protect their vitals from enemy 8 inch shells. Also, their unusual main battery layout and heavy tripod fore-masts made these ships top-heavy and prone to excessive rolling. This combined with low freeboard forward made them inferior seaboats compared to later designs. Rework in the shipyards modified the hull and superstructure in the 1930s to eliminate the rolling. The two vessels in this class, Pensacola and Salt Lake City, were originally classified as light cruisers due to their minimal armor until re-designated in July 1931 as heavy cruisers in accord with international practice of designating all cruisers with guns larger than 6".
Read more about this topic: Heavy Cruiser