Short answers, pulled from the story.
The Galactic Republic lasted over 25,000 years, having been founded when the first Galactic Constitution was signed on Coruscant 25,000 years before the Battle of Yavin. It was dissolved by Supreme Chancellor Palpatine, who reorganized it into the Galactic Empire.
Following the Ruusan Reformation around 1000 BBY, the High Republic demilitarized, retaining only a small token force called the Judicial Forces to protect Coruscant. Member worlds handled their own security, and trade organizations like the Trade Federation actively opposed a centralized Republic military to prevent enforcement of regulations on their business.
The Confederacy of Independent Systems was formed in 24 BBY by systems that seceded from the Republic over grievances including ineffectual government, heavy taxes, and favoritism toward Core Worlds. Former Jedi Master Count Dooku led the Confederacy publicly, though Palpatine secretly directed both sides of the conflict.
Order 66 was a secret command issued by Supreme Chancellor Palpatine that compelled the Republic's clone soldiers to exterminate all Jedi. Inhibitor chips implanted inside the clones' heads ensured compliance, resulting in what became known as the Great Jedi Purge.
Scholars have compared the Galactic Republic's rise and fall to the Roman Republic and Weimar Germany. Silvio (2007) argued the prequel trilogy was also shaped by the George W. Bush presidency, including the Iraq War and the Patriot Act, a reading George Lucas confirmed regarding Padmé Amidala's line about liberty dying with thunderous applause.
After Palpatine's death and the destruction of the second Death Star at the Battle of Endor in 4 ABY, the New Republic was founded as a restoration of the Galactic Republic. It was governed by the former Rebel Alliance, reorganized into the New Republic Military, but eventually collapsed when the First Order's Starkiller Base destroyed Hosnian Prime in Star Wars: The Force Awakens (2015).