Shelby Foote once said that the Battle of Shiloh had more casualties than Waterloo. Then again, the Civil War was full of Waterloo-sized battles. Napoleon and some other generals would have found that impossible to fathom. . . but we shouldn’t be surprised.
Foote offers superb commentary in the Ken Burns documentary The Civil War. (A must-have series for students.) One reason why fatalities were so high, he notes in the series, was because the weapons were ahead of the tactics. Modern guns fired advanced bullets into the sort of infantry formations that Wellington might have used in 1815. It was a similar disaster in World War I, when machine guns, artillery, and gas could erase the charge of a whole battalion. Sometimes even the crudest weapon can neutralize a force: the U.S. Army, which blasted Iraqi armor into slag in the Gulf War, was plagued by pipe bombs after the invasion of Baghdad. These examples reveal a serious flaw in warfare: the successful tactics of a previous conflict are rendered obsolete by the next battle.
Armies find it hard to adapt, but not impossible. The process is just slow. It takes time and effort to retrain a soldier. A considerate commander can find ways around a bloody engagement. If anything, the will to adapt can lower casualties and bring a conflict to a logical conclusion. Perhaps warmongers like Napoleon and Alexander failed in this regard as their armies rampaged over all creation. Wars have to end. The cost could be great, but not nearly as high as prolonged or endless strife.