Gunpowder, which could launch projectiles straight through metal garments, changed that, and with the invention of the tank in World War I, armor become something people rode inside.

The invention of Kevlar in 1965 brought back wearable armor, and then armor-piercing bullets were designed to punch through that.

The next stage of armor might be more of a giant leap than an incremental improvement: a new type of composite metal foam can stop an armor-piercing bullet in just a fraction of an inch.