The Autumn War was a military conflict between Gridania and Ala Mhigo in the years 1468-1469 of the Sixth Astral Era, over 100 years before the events of Final Fantasy XIV. Little is revealed about the conflict in-game, but it had important implications for the realm, as it led to the formation of the first Eorzean Alliance, the creation of a balance of power, and the adoption of gil as a universal currency.
The war grew out of the desire of King Manfred of Ala Mhigo's ambitions, as he hoped to expand his nation's influence by sending forces into the Black Shroud. The Gridanians were unwilling to tolerate this intrusion and, under the banner of the lancer Vainchelon, met Ala Mhigo's forces at the Firesand Banks. The battle of the Firesand Banks was inconclusive, and Vainchelon withdrew, hoping to bait the Ala Mhigan commander, Gylbarde, into overextending into an ambush. However, Vainchelon fell ill and died before his plans could come to fruition.
Vainchelon's replacement, Osbern, ordered a catastrophic frontal assault on Ala Mhigo's forces. Fearing that after the Shroud fell, Ala Mhigo would invade Coerthas next, Ishgard petitioned for an alliance of Eorzean city-states to defend Gridania. Ul'dah and Limsa Lominsa answered the call, and Gylbarde's forces were swiftly defeated.
After Ala Mhigo's defeat, borders were drawn, and it was agreed that no one city-state would be allowed to dominate the others. Furthermore, local currencies had unstable exchange rates and Allagan pieces were outdated, the continent agreed to adopt a universal currency, gil, which depicted the Twelve rather than national heroes. However, the Alliance weakened over time, and in the year 1557, when Ala Mhigo underwent a civil war and was invaded by the Garlean Empire, and Nidhogg awoke and resumed the Dragonsong War by attacking Ferndale, the Alliance did nothing. It would not be until 1572, when the Garlean Empire sought to destroy Eorzea itself, that the Alliance finally united against a common enemy.
Even in the present, the true events of the Autumn War are unclear. According to a journal possessed by Vainchelon's descendant Nourval, allgedely written by Gylbarde, Vainchelon sought peace with Ala Mhigo, and agreed to a secret meeting with Gylbarde to discuss a treaty. However, Gylbarde poisoned Vainchelon at the meeting and sent his son to spread news that Vainchelon had died of illness.
Gylbarde's journal causes some controversy among the Order of the Twin Adder during Operation Rhalgr's Beacon, because the truth (if it is indeed the truth) could stir anti-Ala Mhigan sentiment in the Eorzean Alliance.