The chariot and gunpowder.

The chariot, because it introduced the idea of mobile warriors capable of rapid encirclement, with the concept eventually carried over to regular horseback cavalry.

Gunpowder- Need I even say? As Lord Winter pointed out, the ability to knock out a fortification essentially rendered castles mostly pointless. Additionally, particularly in Europe, the abolishing of knighthood as a prime means of warfare- it was rather pointless to spend years and years practicing swordmanship, lancing, horseback riding, all while encased in a heavy suit of armor, when a lowly peasant could pick up an arquebus, train for a few weeks or a month, and kill you not only much cheaper, but with much less effort. And of course, all of the inventions that would eventually be spurred from guns and gunpowder- cannons, artillary, gatlings, etc...