The Galactic Republic, also known as the Grand Republic or simply as the Republic, was a democratic union of planetary states spread across light-years of space. Old Republic was a term affixed to the pre-modern Republic that existed until 1032 BBY, and the modern Republic that was supplanted by the Galactic Empire in 19 BBY. In addition, the term High Republic denoted the era in which the Republic reached the zenith of its power from 300 BBY to 82 BBY. The Republic was founded many millennia before the Battle of Yavin, approximately 25,000 BBY, around the same time as the Jedi Order. The Galactic Senate governed the Republic under the Galactic Constitution, and comprised thousands of senators representing the Republic’s member worlds. The Supreme Chancellor was the title of the head of the Republic, elected from ranks of the Senate to lead the government on Coruscant. An ecumenopolis or city-covered planet, referred to as Galactic City, Coruscant was the capital of the Republic and therefore the seat of the Senate.
The Republic comprised of various member worlds across the galaxy from the Core Worlds to the Outer Rim Territories, uniting various sentient species, human and alien alike, under one banner. As its power grew, the Republic fought many wars against various opponents such as Mandalorians, the Zygerrian Slave Empire, the Nihil, and several Sith Empires. The Jedi Order, a religious group of Force-sensitives dedicated to the light side of the Force, was sworn to the service of the Senate, and as such the Jedi Knights were hailed as the guardians of peace and justice within the Republic. After centuries of conflict, culminating in the fall of the Old Republic, the last war between the ancient Jedi and Sith concluded with the restoration and rise of the modern Galactic Republic, ushering in the Republic Era which was an age of peace that lasted for a thousand years. The Republic’s history was officially reset, and the ages before this event were merged into a collective whole known as the Old Republic-era.
The Republic entered a period of decline after centuries of expansion and prosperity. Aided by the resurgent Sith Lords, the Separatist Crisis divided the galaxy between the Republic Loyalists and the Confederacy of Independent Systems. The Grand Army of the Republic was organized under the leadership of Jedi Generals who led the Republic Military against the Separatist Droid Army during the Clone Wars. However, the conflict had been orchestrated to allow Supreme Chancellor Sheev Palpatine—the alter ego of Darth Sidious, Dark Lord of the Sith—to control and deceive the Republic in a dictatorial manner. The end of the war saw Sidious revealing himself and his deception, and proceeding to exterminate the Jedi through Order 66, before transforming the Republic into the Galactic Empire and declaring himself Galactic Emperor. The Imperial Senate stood as the last remnant of the Old Republic until it was disbanded by the Emperor at the height of the Imperial Era. In the aftermath of the Galactic Civil War, the Alliance to Restore the Republic was fully reorganized into the New Republic, restoring the democratic system of its predecessor after a generation of Imperial rule.