The Rebellion

"The Rebellion" was an event ocurred 5,000 years ago, when the Crystal Gems rebelled against the Gem Homeworld, to protect the Earth.

History
According to Pearl, in "Rose's Scabbard", the Rebelion ocurred 5,000 years ago, in the Gem Battlefield. She stated in "On the Run" that the Homeworld Gems wanted to do something bad to the Earth, and some Gems such as herself, Garnet, and Rose Quartz found their proposites unfair to the life already existent on the Earth. They rebelled against them, and the historic battle happened with Rose Quartz leading her own armies of Gems, fighting against the Homeworld.

Greg also is shown to know about the war, as he stated in "The Return", that in the war, Rose could only use her shield to save her closest friends, and that lots of Gems were dead, and people too. In this episode, Jasper also stated she was presented in the first war because of the Earth, and that she fought against Rose's armies.

The battlefield was first shown in "Serious Steven", and the events of the Rebellion were most likely recorded on the walls of the Pyramid Temple, as we can see Rose fighting against an unknown Gem.

Trivia

 * The weapons of the defeated Gem warriors were left in the Battefield.
 * In Rose's Secret Armory, we can see an amount of amors, as Pearl said, "The Armor Of The Fallen", indicating those were the armors of the defeated Gems of the war.
 * The Rebellion happened 5,000 years before the events of the show, and at this time, Amethyst was still in the Kindergarten, not being present in the war.