The Rebellion

"I didn't fight a one thousand-year war for this planet's independence to take orders from the likes of you!"

"The Rebellion" or the "Gem War" was an event that occurred approximately 5,500 years ago as confirmed by "Steven Universe: Guide to the Crystal Gems", when the Crystal Gems rebelled against the Gem Homeworld to protect Earth.

History
In "It Could've Been Great", Peridot reveals that the Homeworld Gems planned to colonize the Earth by erecting multiple spires and more Kindergartens, but would strip the entire planet of its resources while virtually erasing all preexisting life on Earth. But Rose Quartz saw that every life was important and worth protecting, and started to rebel against her home planet.

In "The Answer", Garnet revealed that the earth was currently being colonized by Homeworld, under Blue Diamond's jurisdiction. During that time, 5,750 years ago, a small group of rebels emerged to oppose the colonization of the planet. Due to this, Sapphire was summoned to Earth from Homeworld by Blue Diamond. She was assigned three Ruby guards to protect her, one of which included Ruby herself. During a two-Gem raid conducted personally by Rose and Pearl, Ruby accidentally fused with Sapphire to form Garnet for the very first time. After the two rebels fled from the scene, Blue Diamond demanded Ruby to be shattered for the incident. Sapphire reacted by grabbing Ruby and jumping out of the arena towards the Earth's surface. Over an unknown period of time, Ruby and Sapphire developed a romantic relationship and fused once again. After running into Rose and Pearl again in a chance-encounter, Ruby and Sapphire, as Garnet, joined the Crystal Gems' rebellion.

In "Rose's Scabbard", the Crystal Gems visit an ancient Gem Battlefield where Pearl says a battle was fought over 5,000 years ago. Garnet describes the battle as a maelstrom of death and destruction, saying that countless Gems were broken. In "Sworn to the Sword", Pearl later reveals that some of the first battles for Earth happened at the Ancient Sky Arena. She also 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 proposals 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 Crystal Gems, fighting against the Gem Homeworld. Greg Universe also appears to have knowledge of the war; he states in "The Return" that during the Rebellion, Rose could only use her shield to save a handful of her closest friends, and that many Gems and people died during the conflict. In this episode, Jasper also stated she was present during the war for Earth, and that she fought against Rose's armies, enough to develop a level of respect for her tactics.

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

Trivia

 * The weapons of the Gem warriors who were destroyed during the Rebellion were left on the Battlefield.
 * In Rose's Secret Armory, sets of armor are seen that Pearl refers to as "The Armor of the Fallen"; it is possible that the armor belonged to Gems who were defeated during the Rebellion.
 * It is initially implied that Amethyst was about 500 years old during the time of the Rebellion, but it is later stated by Steven that she did not "come out of the ground" until the Rebellion was over, which might indicate that she was still incubating during the event of the war. Amethyst never experienced the war, though she has said that she wishes she could have been a part of it.
 * Greg said the Rebellion ended with Gems and people being destroyed, indicating humans might have been involved in the Rebellion or some humans were unfortunate victims of it.
 * The Crystal Gems used the Wailing Stone during the Rebellion to communicate, as confirmed by Pearl in "Rose's Room".
 * Pearl has stated that the Rebellion lasted roughly one thousand years.