The Galactic Empire, declared as the First Galactic Empire, commonly referred to as simply the Empire, and remembered as the Old Empire, was the fascist galactic government that replaced the Galactic Republic in the aftermath of the Clone Wars and ushered galactic civilization into a new age.
The Empire, also known as the New Order, was founded and ruled by Galactic Emperor Sheev Palpatine, who was secretly a Dark Lord of the Sith called Darth Sidious. The new regime rose to power with a groundswell of populist support, promising peace and stability from the destructive Clone Wars and capitalizing on anti-Jedi and anti-droid sentiment and human–ethnocentrism. The subsequent Age of the Empire would last only two decades in contrast to the centuries of the Republic Era, but not before the Jedi Order was systematically purged and almost eradicated, and the galaxy largely conquered and oppressed.
The Empire stood virtually unchallenged until numerous insurgent cells united to initiate the Galactic Civil War, with the growing rebellion winning its first major victory over Mustafar in 4 BBY. These rebel cells formed the Alliance to Restore the Republic in 2 BBY, and the Rebel Alliance itself achieved its first major victory with the theft of the Death Star plans on Scarif which, in turn, led to the destruction of the Death Star at the Battle of Yavin in 0 BBY. The following years saw initial Alliance successes until a major Imperial victory on Hoth in 3 ABY, but the Empire suffered a devastating defeat one year later at the Battle of Endor. During the battle, Emperor Palpatine was slain at the hands of his Sith apprentice, Darth Vader, who also died shortly thereafter.
With the Emperor dead and the Sith Lords extinct, factionalism overcame the Empire as it fractured into fiefdoms led by Moffs, admirals, and generals, many of whom ignored Grand Vizier Mas Amedda‘s authority at the capital and became warlords. Loyalist forces of the Imperial Navy came to be publicly led by Grand Admiral Rae Sloane and covertly ruled by Fleet Admiral Gallius Rax and his Shadow Council. Ultimately, Rax seized control over their forces, removing Sloane from power and becoming Emperor-in-effect through his new position as Counselor to the Empire. Under Rax’s command, the Empire made its final stand at the Battle of Jakku in 5 ABY, ultimately losing to the New Republic. While several Imperial holdouts continued to exist, the Empire formally capitulated to the New Republic with the signing of the Galactic Concordance.