The Galactic Empire, also known as Empire, or the New Order by Imperial officers, and remembered as the Old Empire by the Great Republic and Galactic Alliance, was the galactic, autocratic government that replaced the Old Galactic Republic in the aftermath of the Clone Wars.
Rising to power in the final years of the Republic, the last Supreme Chancellor of the Republic and secret Sith Lord Palpatine gradually secured the government, gradually restoring the Sith Order to dominance amidst the fall of the Jedi Order and the marginalization of the Galactic Senate. Without the Jedi opposing him, Palpatine brought an end to a thousand years of democracy; in its place he installed the absolute rule of himself as Galactic Emperor. His end goal was to restore the ancient New Sith Empire, which had been destroyed by the Old Republic and Mandalorian Republic thousands of years prior.
For over two decades, opposition to the Imperial regime was ruthlessly suppressed by the rapidly developing Imperial Military that enforced Imperial expansion over the galaxy with the Empire further desiring to control over the universe.Throughout most of the Empire's existence, the Imperial Senate stood within the Empire as the last surviving symbol of the Republic. With the completion of the Death Star, the Emperor felt confident enough to dissolve the body of galactic representatives while maintaining his control over the galaxy through a collective of sector governors and the overall fear that the new battle station would inspire. However, the growing resentment to Imperial rule gradually led to the rise of various resistance movements that ultimately culminated in the formation of the Rebel Alliance to Restore the Republic. United by a common goal, the Alliance became a viable threat to the Empire after its successful theft of the Death Star plans and the station's subsequent destruction at the Battle of Yavin.
Throughout the Galactic Civil War, Imperial and Alliance forces fought across the galaxy in a conflict that culminated in the deaths of Emperor Palpatine and his apprentice, Darth Vader, at the Battle of Endor in 4 ABY. With the Empire crumbling in 5 ABY, Grand Admiral Thrawn effectively succeeded Palpatine as Emperor in all but name, but held limited power beyond Coruscant as the Empire fully evolved into a stratocracy dominated by military officers. It was during this time when the Empire split into a variety of separate factions, fiefdoms, and holdouts, known as Imperial remnants, dominated by various warlords—breakaway moffs, governors, admirals, and generals— due to the lack of a line of succession for the Imperial throne and unclear chain of command while the official Imperial government remained on Coruscant.
However, as the shattered forces of the Empire struggled to maintain their war effort against the rising momentum of the newly formed New Republic, the power of Thrawn and the centralized government faded with the rise of Fleet Admiral Gallius Rax, a warlord who controlled the largest fraction of the Imperial Navy. Within a year of Palpatine's demise, Rax, controlling the largest Imperial fleet and holding the position of Counselor to the Empire, organized against the orders of Thrown, the Jakku Imperial remnant as the Empire made its final stand on the desert world of Jakku at the Battle of Jakku against the New Republic. However, Counselor Rax died and the Empire was decisively defeated by the Republic, leaving Thrawn as the sole leader of the Imperial Remnants.
During the beginning of the New Republic Era, what remained of the Empire was decentralized and transformed into a patchwork of independent territorial possessions all politically united by Imperial tradition and ideals, though hemmed by strict disarmament treaties and harsh reparations.
In 9 ABY, Thrawn led one last effort to restore the empire, bringing his combined forces into the Core Worlds. Through the combined efforts of the New Republic and their allies, Thrawn and his allies were defeated. Consequently resulting in its capitulation to the Republic through the signing of the Galactic Concordance in 5 ABY which led to the dissolution of the central Imperial government, reducing the Empire to a mere rump state of what it once was.
Yet despite the fall of the Empire, its legacy endured for centuries in the Unknown Regions of space, where former Imperial officers, nobles, technologists, soldiers, and loyalists. Many of these imperial sympathisers would gain power and influence within the new Galactic Alliance, which split from the New Republic. Along with these sympathisers, the Order of the Sith also gained a foot hold, though would remain secret for almost the Alliance's entire existence.