It still shocks me that no one person predicted the carnage of the American Civil War. It shocks me even more than in WWI, the Civil War was studied but not applied to the problem at hand.
The generals of the Civil War studied Clausewitz and Napoleon, and the troops went to war with smooth-bore bronze cannon and Napoleonic fire drill. They were largely tempered in the brief, not-too-bloody, and glorious Mexican-American War. It took them a few years and tens of thousands of casualties to learn what havoc accurate, long-range rifles, even if muzzle-loading, could do.
Militaries are eternally building themselves to fight the last war. When they do learn from their experience, quite often the lessons learned are the wrong ones.
The US has always had a charming dread of standing armies.