Why upper belt armour appeared and disappeared

Did Britain and other navies make a mistake by adding upper belt armour after Dreadnought and Invincible omitted it?

Not necessarily; upper belts answered contemporary expectations about plunging fire before protection shifted toward armoured decks and integrated citadels as gunnery and design evolved.

Upper belts answered contemporary expectations about shell trajectories as plunging fire became a greater concern. Protection subsequently shifted toward armoured decks, sloped sections, and a more enclosed protective box. Rapid construction gave designers more opportunities to develop and implement changes, so successive solutions appeared within a short period. The simultaneous construction of Indefatigable and Lion illustrates the resulting awkwardness: one class was already overtaken by another being built beside it. Dreadnought and the Lord Nelsons provide a different comparison, since completing the older design supplied insurance beside a one-off experimental step. In this interpretation, the Indefatigables offered no equivalent safeguard.