The power of those espers... I had no idea.
The Battle of Vector is a decisive battle between the Gestahlian Empire and various anti-Imperial forces, in which the Imperial capital of Vector is assaulted from two angles. Though the espers wreak havoc on Vector and Returners and Narshe Guards occupy the burning city without encountering any resistance, the anti-Imperial forces fail to end the war as the situation escalates and the Empire seizes the offensive again after a short ceasefire.
Coalition battle plan
Now we are left with no choice but to take up arms against the Empire. I suppose this was inevitable...
After the Returners raid the Magitek Research Facility in Vector, the Empire loses access to magic, evening the playing field and making a direct assault on Vector more feasible. The Returners intend to use Kingdom of Figaro's machines and Narshe's resources to attack Vector. The Elder of Narshe agrees to support the Returners, but the coalition lacks the manpower such an offensive would require. A plan is made in which Terra Branford (being half human and half esper) crosses into the Esper World negotiating with the espers to organize a joint offensive in which a coalition of Returners and Narshe Guards will attack from the north, while the espers will strike from the east. The desired result is not only an end to the Gestahlian Empire's aggression against the rest of the world, but also the restoration of harmony between humans and espers who have been considered incompatible since the conclusion of the War of the Magi.
18 years ago, Imperial soldiers invaded the Esper World and took several espers captive, but were thrown out of the Esper World as the espers closed the gate between the two realms, making the Esper World inaccessible to humans. Kefka Palazzo plans to allow Terra open the gate so the Empire can fulfill the task they failed at 18 years earlier to capture the remaining espers and drain them from their powers. Troops are withdrawn from the Imperial Observation Post so that the Returners can reach the gate, and as Terra and her friends move on, Kefka follows closely behind.
When Terra opens the gate to the Esper World it becomes apparent that both sides' planning had been flawed; while the Empire had miscalculated the espers' power, the Returners had failed to foresee the reaction of their would-be allies as the gate opens. Espers flow out and attack everything in sight, including both Imperial soldiers and Terra's allies. The espers head towards Vector, intending to free their friends who were captured in the previous Imperial incursion into the Esper World. When they discover the prisoners are dead, they decimate the city before flying away. Helplessly watching the destruction of their capital, Imperial soldiers flee in terror to the Imperial Palace.
When the Returners and the Narshe Guards move into Vector they are shocked by the destruction. Narshe Guards later tell Locke that the Imperial Army had already fled when they arrived and no combat had been necessary.
Emperor Gestahl surrenders to the Returners, who in turn agree to help extending the ceasefire to include the espers. The Empire has not given up its goal of acquiring more magicite and just as General Leo of the Empire and Yura of the espers reach a ceasefire in negotiations in Thamasa, Kefka attacks the town. In a one-sided battle he kills the espers and enters the Esper World with Emperor Gestahl, restarting the war and raising the Floating Continent. The playable Returners who remained in Vector narrowly escape an Imperial trap, and neither Banon nor Arvis are ever seen again (though it has never been confirmed they were killed). Vector is completely abandoned following the rise of the Floating Continent, and all occupying troops seem to have withdrawn.