Ok I'll bite.
Battle of Adrianople 378 - heavy cavalry for the first time displaced heavy infantry as the prime influence on the battlefield. The beginning of the end of the dominance of the Roman Legion.
Battle of Agincourt 1415 - the English Longbow for the first time disputed the role of the armoured knight as the dominant force on the battlefield, also allowed a far inferior force to defeat a numerically superior force with relative ease.
WW1 - introduction of the tank was one of the major reasons why the stalemate of 3 years of war was broken. Germany, usually well to the fore of military technology, was caught completely by surprise and had little answer.
WW2 - advent of the ground attack aircraft. Employed by all sides in the conflict this variant of air power came of age and was instrumental in both the early German Blitzkrieg victories and the later Allied triumphs.