The Galactic Republic was the ruling government of the Universe for 25,034 years, until the establishment of the Galactic Empire in 19 BBY. Although it was commonly known as the Old Republic, this was not so until after the establishment of the Empire; it's name when it existed was the Galactic Republic, or simply The Republic. Rarely it was refered to as The First Republic. At times, the term Old Order has been used to describe it.
The Republic, led by a Supreme Chancellor, had a supposedly democratic government that was able to sustain itself for more then twenty-five thousand years. For most of it's duration as the dominant Universal power, the Republic did not style itself as a strong, centralized, and highly organized government, but rather as a union, alliance, or federation of dimensions for the purposes of promoting commerce, meditating disputes, and providing mutual protection. It is said that it's complete history would fill "a thousand libaries".
In it's latest years, it was a benvolent-though ineffectual-government tied up in layers of bureaucracy.
The Republic was born with the signing of the Universal Constitution in 25,053 BBY, during the Unification Wars. During that time, the Humans and Vulcans, reverse-engineering the Force-based technologies of the Rakata and the energy-based technologies of the T'Kon, invented the hyperdrive, allowing Coruscant to become the capital of the Galactic Republic 53 years later and would remain that way for twenty-five thousand years. The Republic was created by the Core Founders. Unbeknownst to even the Jedi Masters, the GenoHaradan began to secretly protect the newly formed universal government.
Shortly after the formation of the Republic, the Perlemian Trade Route was mapped, linking Coruscant in Star Wars to Betazoid in Star Trek, and bringing the Jedi Knights into the Republic. Over the next millennia, the Corellian Run was mapped, linking Coruscant to Corellia, within Star Wars, and beyond. The wedge defined by the two trade routes became known as "the Slice." Since the stretch of hyperspace universal-west of Coruscant and Star Wars was anomaly-ridden and impassible for then current Republic technology, the government expanded eastward, instead. The Core Dimensions' portion of the Slice was known as the Arrowhead, and soon the Republic expanded into the Action Dimensions.
Though history often recalls the Republic as peaceful; numerous universal conflicts, such as the Hundred-Year Darkness, the Old Sith Wars, the Vultar Cataclysm, the Mandalorian Wars, the Jedi Civil War, the New Sith Wars, and the Clone Wars—occurred throughout its 25,034 year history.
At first, the government on Coruscant wished for slow expansion, despite its promotion of hyperspace exploration. However, against the government's wishes, the membership of the Republic boomed for its first millennium of existence, mainly due to worlds and dimensions wishing to join as protection against the Hutt Empire and the Borg Collective. In 25,000 BBY its capital had officially become a universal hub.
For years, the Jedi Knights and the ancient armies and navies of the Republic defended against violence, both within and without. Around 24,500 BBY, the First Great Schism occurred, after the Legions of Lettow were established by dissident Jedi who believed that true power was achieved not through passive meditation, as taught by their Jedi Masters, but through emotion. The tension between the Jedi and these dissident Fallen Jedi, led first by Xendor, and then after Xendor's death, by Arden Lyn, grew until conflict erupted.
In 24,000 BBY, the Republic entered into the Borg War against the Borg Collective, in the north-eastern Star Trek dimension. The war traveled up and down the Perlemian, and the respective capitals of Coruscant and Borgia both suffered volleys of atomic pressure bombs. The Republic eventually won the war by stirring up the Hutts against the Borg. Within a century, most of the Borg region was incorporated into the Republic as a dependent governorship.
During the Great Manifest Period (20,000-17,000 BBY), the Republic expanded eastward across the Universe, especially in the Historical and Comedy dimensions. Despite this, the Slice would remain the heart of navigable space. Eventually, the Republic expanded past the universal-northern and -southern borders of the Slice, though most of the Universe still remained undiscovered, except such outposts as the Transformers dimension (10,000 BBY) and the Judge Dredd dimension (8,000 BBY). The space west of Star Wars still remained unexplored since analogues to the Perlemian Trade Route and the Corellian Run in that direction still had not been discovered. Worse, progress was limited to dangerous one- or two-light-year jumps into a stretch of hyperspatial anomalies. During this period, the Criminal Dimensional Region was formed. However, this region suffered greatly as a result of the Alsakan, in Star Wars, attempting to snatch control of the Republic from Coruscant. This resulted in seventeen Alsakan Conflicts, spanning from 17,000 BBY to 3,000 BBY. However, most of the fighting took place between their holdings in the Crime Dimensions, rather than in Star Wars and Star Trek themselves. Needless to say, Coruscant won every conflict it fought.