|YOU'VE FORGOTTEN YOUR PURPOSE!
The Rebel Army is a term for large insurgent forces gathered together to resist Aku. The first rebel army's main goal is to try to end his reign of terror on Earth while the second rebel army's goal was to rescue Jack. They appeared in "Episode XCVI" and "Episode CI".
Fifty years after Jack arrived in the future, The Scotsman started to gather an army to battle Aku. The army included forces from at least three separate tribes or groups mobilizing together, evidenced by the army being composed of three distinct forces: a large group of men riding tanks, another group riding on rhinos and the Scotsman's own daughters marching on foot. The Scotsman and the combined armies under his command were eventually able to discover Aku's palace, with the Scotsman delivering the order to attack soon after. In response to this uprising, Aku erupted from the tower in the form of a gigantic black ball, rolling over the tank operators and the rhino riders with his mass. This prompted the Scotsman to order his daughters to retreat while Aku was distracted with the other two groups.
After annihilating the cavalry and tank divisions, Aku then decided to half-heartedly pursue the retreating girls to make it "a complete annihilation", but the Scotsman was able to intervene and distract Aku long enough for them to escape. However, the Scotsman's bravery and wise-cracking ended up resulting in him being targeted with a powerful blast from Aku's eyebeams. The girls, having witnessed their father's death, mourned for his supposed loss, only for him to return as a ghost through Celtic magic, specifically the runes engraved upon his now-shattered sword. After being resurrected as a ghost, he then told his daughters that they would raise another larger army, as well as seek out Jack, before returning to fight Aku once more.
When Jack was captured and Aku broadcasted the news live around the world, an army of those who were once saved by Jack arrived to rescue him.
It is unknown how Aku's destruction in the past affected their existence. Not to mention since there were no Aku for them to rebel to.