It had been several years since Aizen was in control of Hueco Mundo. Without him or his Espada, the Hollow world was in an almost constant state of civil war and strife. Akujin, the acting Captain-Commander of the Gotei 13, secretly lead several secret voyages into Hueco Mundo to establish his Arrancar Five. One day, during these escapades, he went "missing", forcing his aunt, Amatarou Omaha to take over as Captain-Commander. However, Omaha felt something was amiss and assigned both the Tenth Division and the Eleventh Division to investigate his supposed disappearance into the Hollow world.
The Arrancar were the ones to launch the first attack. They appeared in Rukongai, led by Gyoumaru and Rascar, and proceeded to attack, only to have their attack halted by then-lieutenant of the Eleventh Division, Kakashi Hikaru. Kakashi's defense caused the Arrancar to flee, and brought the new Arrancar threat to the full attention of the Gotei 13.