Torpedoes promise a cheap decisive weapon

With armour blunting the earlier shell-based idea, torpedoes appeared to offer a fresh way for small craft to sink expensive ships. French enthusiasm for the weapon encouraged early submarine development, with the submarine understood as a submersible torpedo boat that could extend torpedo reach. The advantage was always contingent: rapid-firing guns could force surface torpedo boats to manoeuvre or destroy them before a reliable attack. An offset built on technology has to keep evolving as opponents develop counters instead of assuming one weapon will settle the contest permanently.