The US Navy's 21-knot battle-line trap

The 1916 Daniels plan promised parity with Britain by 1924, but historical construction rates did not match that ambition even after the United States entered the war. Congress also left the navy short of cruisers, destroyers, submarines, and crews, producing an unbalanced force around its battleships. Pennsylvania, New Mexico, Tennessee, and Colorado designs retained powerful guns but became tactically vulnerable once foreign battle lines approached 28 knots. Speed would let an opponent choose range and bearing, so turbo-electric machinery alone could not rescue the 21-knot standard without a much larger hull and power plant. Guns and engines were the expensive parts of a capital ship, making a fast redesign potentially cost about twice as much and inviting a congressional construction pause before a new battle line emerged.