Honestly I would have to say gunpowder and the tank as the defining moments when the nature of war changed.
it used to be a battle of generalship and training of individual soldiers that copuld make or break a battle but with the inclusion of guns and tanks into a battle it became more of a give the body a gun or give the dozer driver a tank and you suddenly have a semi effective fighting force.
the list is from post WWII until present