"In 1930 Japan signed the London Naval Treaty, imposing limits on cruiser, destroyer and submarine tonnage at the same 10:10:7 ratio to which the United States, Britain and Japan had agreed in 1922 for battleships. Cruisers armed with 8-inch guns and those with 6-inch guns fell into separate categories, and so all three navies planned new classes of light cruisers armed with 6-inch guns. Size limits remained the same as for 8-inch cruisers, and so these new "light" cruisers would be as large as their "heavy" counterparts.
The United States built its Brooklyn class with fifteen 6-inch guns in five triple turrets, and Japan planned its own Mogami class with a similar armament. Both navies also intended to replace the turrets with new ones each carrying two 8-inch guns at some future date, but in the United States this remained only a paper exercise. Japan went much further.
Mogami, the lead ship of the class of four, was laid down in late 1931 as an 8,500-ton light cruiser with fifteen 6-inch guns and a designed speed of 37 knots. To meet the nearly insane targets for speed and armament on such a small displacement, Japanese designers employed extensive electric welding techniques, and gave her very little armor protection…"
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