We are an Empire ruled by the majority! An Empire ruled by a new Constitution! An Empire of laws, not of politicians! An Empire devoted to the preservation of a just society. Of a safe and secure society! We are an Empire that will stand for ten thousand years!
—Palpatine
The Galactic Empire, now known as the Old Empire, was an organization formed from the Old Republic in 19 BBY after the purging of the Jedi Order and the Declaration of a New Order. The Empire became an evil symbol of fear, power, control and tyranny as Emperor Palpatine's alter ego–Darth Sidious–had full control of his body, bringing about more oppressive rule in the galaxy.
Eventually an underground alliance known as the rebellion was the first to challenge the Galactic Empire head on by performing series of hit-and-run tactics against the Empire as well as performs said tasks to retrieve numerous prisoners, captured innocent families and people. One notable group of the rebellion was the crew of the Ghost, a small brave group of rebels who have originated from the planet of Lothal. Together this group would be essential in formation of the more organized Alliance to Restore the Republic who then started to gain attention from citizens of the galaxy.
These citizens, people and various groups of other rebel cells made up the Alliance Army. With the growing numbers of rebellion against the Empire, officials within the Imperial Military then decided to use their new found power, the Death Star–using it to decrease the morale of the rebel soldiers and succeeded–but before they could start using it more, it was destroyed and a few weeks later, started construction on another Death Star.
The Imperial Navy started to increase their production rate of their impressive amount of warships known as Imperial-class Star Destroyers, the evil once Jedi Knight now a Dark Lord of the Sith Darth Vader stumbles upon the Rebel's hidden base on the cold planet of Hoth, he leads a daring assault to flesh out the rebel terrorists. Assaulting and destroying the rebel's base, Vader then continues the search for the boy who destroyed the first Death Star and attempt to turn him to the Dark side of the Force.
Finally a year after the Battle of Hoth, the Empire's Death Star II–bigger and more powerful than the first–is nearing its completion and plans to use it to end the Rebellion once and for all. After a few more days, the Alliance then makes their supposed surprise attack on the battlestation only to find themselves completely trapped by a massive imperial fleet on the far side of the forest moon Endor.
Having underestimated the Alliance's chances, the Empire then take a massive blow to its military structure with the destruction of their Emperor as well as the Supreme Commander Darth Vader. Not only the Empire fractured, but the Alliance as well with the death of their Chief of Staff who was assassinated by a group of bounty hunters posing as her guards, resulting in both organizations fracturing. The battle then became a tie as various admirals and commanders of both navies and armies scatter, and taking each other out at the same time.
Three weeks after infighting of the fractured Alliance to Restore the Republic and Galactic Empire, an unknown, mysertious being only known as Angel reveals to every known warlord, commander and officer of both broken organizations that if they continue their fighting, the peace the galaxy needed wouldn't exist and in order for peace to come is to unite together as the Imperialist Alliance. Believing this Angel to be legitimate, all surviving forces of the Alliance and Empire then stop their fighting and agree to come together as the Imperialist Alliance.
For two months and a week, the Imperialist Alliance keeps order in the New Galactic Governmant, but Angel then pushes his "phase two" and has every known leader of the Imperialist Alliance assassinated and replaced with his clones, eventually reorganizing the Imperialist Alliance as the Galactic Eclipse Empire, enforcing his absolute rule over the New Empire.