Little is known of the First Age, except that the land was full of mythical races such as dwarves, elves, trents, Atlantians and more. One of the only historical events to be recorded is the discovery of the Nether Portal. Once the first one was created by the Elven King Jak'lyc, the Nether War was an inevitability. The Zombie Pigmen have no motivation except the absolute destruction of the over world. Their motives behind this are unclear, as they can't or won't communicate to other races, except their Blaze Overlords.
At first, people and Elves, working together, tried to exploit the Nether, and harvested an enormous amount of glowstone to light up the world, but the skirmishes led to battles, which in turn, led to total war. The Second Age was one dominated with war between the Zombie Pigmen and the races of the overworld. This war eventually culminated in all races except humans leaving the Realm of Vasten and the portals being closed down.