Prior to Tidus appearing in Spira, plans were already being made for a coalition of Crusader and Al Bhed forces to attack Sin through highly developed machina technology, a tactic which had previously been disregarded. While clearly in defiance to the teachings of Yevon, the operation was initially discouraged by various ranks of the clergy, but was ultimately supported by the newly appointed Maester Seymour Guado as well as Maester Kinoc. Knowing full well that the Crusaders and Al Bhed forces would be decimated, the church supported the endeavor, so as to even out the population of the non-faithful, once again using Sin to solidify their status at the top of society.
The Mushroom Rock Road is made the base of operation for the upcoming battle, the Crusaders capturing Sinspawn while the Al Bhed set up machina weapons up with a large railing gun among them. Once Yuna and her guardians arrive at the command post, invited to see the operation unfold, they end up fighting Sinspawn Gui, a conglomeration of the many Sinspawn that the Crusaders had gathered into a cage that they suspended to attract Sin.
As Sin speeds toward the shores, the Crusaders bombard it with cannon fire while Al Bhed specialists ready their machina. The attacks shed Sin's outer layer into Sinspawn that detach from the main body and engage the Crusaders and Chocobo Knights. Sin destroys the Crusader infantry with its Giga-Graviton, and although this gargantuan field of energy is held back by the Al Bhed machina gun at first, Sin overcomes it and destroys them as well. With most of the ground troops dead, Yuna and Auron aid Seymour in defeating Sinspawn Gui.
Having suffered excommunication from Yevon for the attempt, many of the surviving Crusaders retire and begin to work for temples of Yevon, including the nearby Djose Temple, to seek forgiveness. Some of the other survivors, believed dead, were found by Leblanc and became members of her syndicate. The machina gun that was destroyed by Sin was later salvaged by the Youth League to serve as their base so their headquarters can double as a memorial to those who lost their lives in Operation Mi'ihen.
- The player's actions determine whether Gatta or Luzzu dies during the battle. If the player encourages Gatta to go to the front lines twice, he will be killed, but if any other situation occurs, Luzzu will be dead in his place. Thus, Luzzu's death is the default option.
- As revealed in Final Fantasy X-2 Ultimania Omega, Wen Kinoc planned to have the Crimson Squad investigate the Den of Woe under the Mushroom Rock Road the same day Operation Mi'ihen took place to cover up any casualties, as he knew Operation Mi'ihen would be a disaster.
- Final Fantasy X-2 Ultimania Omega p080