There are many candidates:
- the gladius, which enabled the Romans to use swords in tight formation (in comparison with the greek phalanx, which used spears)
- the trebuchet, which meant that besieging armies could take down the walls of a castle rather than starving out the garrison
- the socketed bayonet, which meant that musketeers could defend themselves against cavalry, rather than needing pikemen to do it
Of all the choices, I'd say the one to go with is the aircraft.
Whether dropping bombs on cities, flying men and material behind the lines, strafing ground forces, or sinking battleships, it has changed conflict in almost every way. When we took to the air, war became a different thing.
|