During the American Revolution, the British came prepared to fight a successful war—but against a European army. Their formations, which gave them devastating firepower, and their red coats, which emphasized their numbers, proved the exact opposite of the tactics needed to fight a guerrilla war.

Before World War I, generals still saw the cavalry as the flower of their armies. Of course, the horse soldiers proved worse than useless in the trenches.

Before World War II, in anticipation of a German attack, the French built the "impenetrable" Maginot Line. History repeated itself and the attack came, but not in the way they expected. Their preparations were useless because the Germans didn't attempt to penetrate it; they simply went around it, and France was defeated.