The Valyrian-Ghiscari Wars were a series of five great wars fought roughly five thousand years ago in which the upstart Valyrian Freehold clashed with the ancient Ghiscari Empire to determine which would be the dominant power in the continent of Essos.
Five thousand years ago, when the Valyrians discovered and tamed dragons and began to conquer outwards, the Ghiscari Empire sought to put a halt to their expansion. The two great rivals vied to be the dominant power in Essos, and in a series of five major wars the massive Ghiscari legions marched against he Valyrians, but each time they were defeated by the Valyrian dragons. Much of the fighting took place in and around the Valyrian Peninsula and (what would later become known as) Slaver's Bay.
The Valyrians finally defeated the Ghiscari Empire when their armies and their dragons attacked the Empire's capital city of Ghis. The buildings and streets were burned to ash, and the Valyrians sowed the earth with salt so that nothing would grow again. Five thousand years later, Old Ghis is still a ruin.
In the books
In the A Song of Ice and Fire novels, the five massive wars between the Valyrian Freehold and Ghiscari Empire paved the way for five thousand years of Valyrian dominance in Essos, though it would be another four thousand or so years before they began expanding into the area of the Rhoyne River and modern Free Cities, displacing the Rhoynar people, whose refugees fled to Dorne.
The rivalry between the Valyrian Freehold and the Ghiscari Empire seems to be a narrative echo of the Punic Wars between Rome and Carthage, particularly how the Valyrians sowed the fields of Old Ghis with salt so that nothing would grow again.