The French Revolution.

The machine gun caused trench warfare to be a necessity as 90%+ of the casualties of WW1 were from this weapon. The use of Napoleonic tactics against new technologies was disastrous.

Trench Warfare, tanks, and improved firearms. Essentially, it was non-napoleonic warfare.

The Napoleonic Code.

The three main formations used in Napoleonic Warfare were the Line Formation used when approaching the enemy in which most armies would fight, the Column Formation used when armies were marching and usually not used in battle as only the first few ranks of men could fire (although Napoleon preferred his armies to fight in this formation), and the square formation which was when an infantry company would form into a giant hollow square in which to ward off enemy cavalry attacks.