The Machina War (マキナ戦争, Makina Sensō?) was an event that occurred one thousand years prior to the events of Final Fantasy X. It was a war waged between the city of Bevelle and the city of Zanarkand. The war ended with the creation of Sin, causing death and sorrow to rain down on Spira for the next 1,000 years.
As told in Final Fantasy X-2.5 ~Eien no Daishō~, prior to and at the time of the war, Bevelle was under the rule of a polytheistic religious authority, who, for the most part, had control over magic. The people with predisposition to wield magic were at an advantage to those who could not. One day, a tribe of workers called the Bedore invented appliances dubbed "machina" that achieved things that were previously only possible with magic, turning the tide and displeasing the authorities, who lost their leverage over the common folk. Instead of demonizing the Bedore and their machina, the government elected to find a way to take advantage of them.
The Bedore became increasingly audacious with their inventions, and the government became more and more tyrannical. Not only the Bevelle government, but most city-states, had their own Bedore workshops developing machina, giving rise to the machina cities. The Bedore didn't have the option of refusing to build further machina, or else they would be executed.
The summoner at the head of Bevelle declared the Mage of Zanarkand, his daughter and his followers as heretics for straying from the path of their gods. Bevelle thus attacked, sparking the war, but hadn't excommunicated the "heretics," counting on their charisma to win the war and wanting to use them until the very end. Bevelle hoped that, this way, Zanarkand would give up on using machina weapons against them. The worst fear of Zanarkand wasn't the machina wielded by Bevelle, but the summoners, and they sent assassins to the base where they gathered, aiming to exterminate them using bombs that resembled blitzballs.
The leader of the Bedore, Alb, had a mechanical footsoldier project he called "Fake Bedore" that he wanted to substitute human infantry with. The development wasn't going as quickly as needed and Alb, in his cowardice, sought to fool his own allies and used real human Bedore fighters covered in yellow suits and gas masks. They wielded long wire-like whips and were weak, lacking any battle prowess due to no prior training. The first "model" shown to Velm was scared and spilled blood when struck by a sword. During the battle with the assassins from Zanarkand, Velm realized no such thing as fake Bedore existed, and that they were human units.
Bevelle's technological prowess culminated in a weapon dubbed Vegnagun, which was stored away underneath the city because it was deemed unable to distinguish friend from foe. A warrior from Zanarkand, Shuyin, infiltrated its lair and tried to operate the machina, but was stopped by his lover, summoner Lenne, before both were gunned down by Bevelle forces.
Bevelle had the upper hand over Zanarkand. Knowing this, the ruler of Zanarkand, Yu Yevon, had the remaining summoners and the townspeople who survived the war become fayth-with the exception of his daughter, Yunalesca, and her husband, Zaon-to summon a memory of his beloved city. Using the pyreflies of the dead soldiers, he created Sin, which destroyed the real Zanarkand. As the Bevelle forces reached Mt. Gagazet, they witnessed the ruins that had been Zanarkand, the multitude of fayth that were atop the mountain singing the "Hymn of the Fayth", and Sin looming over the horizon. By the time word reached Bevelle, a ceasefire was called and the war ended.
Bevelle surmised that the fayth on Mt. Gagazet were the people of Zanarkand and that they were being used by Yu Yevon to summon Sin. Yunalesca claimed to know how to defeat Sin, but in exchange she expected the people to revere her father, giving rise to the temples of Yevon. She and Zaon used the final summoning to defeat Sin. She remains in the ruins of Zanarkand as an unsent, waiting for others who would follow in her footsteps. Sin reemerged shortly after, restarting the spiral of death.
The Bedore, who were now known as the Al Bhed (a bastardization of Alb's Bedores), were blamed for the war and hunted down. For the next 1,000 years, the Al Bhed would live in separate communities, detested by the followers of Yevon.
Machina is the Latin word for "machine".