The armor limit behind the battleship's decline

What ultimately makes the battleship concept untenable?

It fails when no practical armor can withstand the first hit from contemporary weapons, leaving firepower greater than protection.

The Second World War was not a simple death knell for the battleship. Part of the post-war pause was strategic: after Vanguard and the Iowa class, the Soviet Union did not create an opposing battleship programme that demanded a Western reply. Continued Soviet construction could have prompted further British and American battleships. The deeper technological boundary arrives when no practical armour can resist the first impact from contemporary weapons. Once protection cannot absorb that opening hit, designers must depend increasingly on firepower and speed, producing ships that behave more like battlecruisers than traditionally protected battleships.