Obviously, the clones (born in a test tube and bred with accelerated aging on Kamino) were first introduced in the title-appropriate Attack of the Clones film, helping the Republic and Jedi fight off the Separatists on Geonosis.
The origins of the clone troopers date back to the days before the Invasion of Naboo in 32 BBY, when Jedi Master Sifo-Dyas, convinced that the Galactic Republic would soon face an existential threat, secretly commissioned Kaminoan scientists to create an army for the Republic.
Representing the future of galactic warfare, clones were designed to be far superior to battle droids. During the last years of the Galactic Republic, clones formed the backbone of the Republic Military, which waged war against the droid armies of the Confederacy of Independent Systems.
Clone troopers were grown on the planet Kamino from the genetic template of bounty hunter Jango Fett, and made into a highly-efficient military force. The origin of their creation was murky; Prime Minister Lama Su told Obi-Wan Kenobi that the order was placed by Jedi Master Sifo-Dyas in the name of the Republic.
The Army the Republic Used Before the Clone Army [Legends] - Star Wars Explained
What did the Republic have before clones?
As the early Republic Armies consisted of local militias and mercenaries with no true command in place, its generals often received orders straight from the Republic Defense Ministry, and no single surface marshal coordinated or directed its operations.
Then, in Revenge of the Sith, the clones did an about-face, slaughtering Jedi after Order 66 — an embedded protocol that forced the clones to carry out the executions — was implemented. However, when the action picks up 19 years later in A New Hope, the clones are mysteriously gone, replaced by human stormtroopers.
In Guide to the Grand Army of the Republic and the novel Republic Commando: Triple Zero (both written by Karen Traviss, with Ryan Kaufman as co-author on the Guide…), there were 3,000,000 clones one year after the Battle of Geonosis.
The Republic had no real standing army for 1000 years, and relied entirely on the Republic Judicial Forces, and in comparison to the armies and navies fielded before the Ruusan Reformation, and the armies & navies from the Clone Wars on the Judicial forces were kinda like toddlers playing soldier.
In G-canon the difference between the Old Republic and the Galactic Republic was addressed in the second season of the TV show Star Wars: The Clone Wars, making reference to a fallen "Old Republic", in which the Jedi Order was also present, predating the Republic of the Prequel-era.
According to Imperial Estimates in Star Wars Inside #87, only about 100 of the Jedi survived Order 66. Most were subsequently hunted down and killed in the years following the Great Jedi Purge.
After all, the Republic's use of clones instead of the droids was a part of Darth Sidious' grand plan; a Trojan horse that toppled down both the Jedi Order and the Republic. Had the Republic used a droid army instead, Order 66 could have been revealed earlier, during a routine checkup.
At the time, many clones did not see it this way, due to the inhibitor chip forcing their hand in destroying the Jedi. However, Star Wars has made it clear when focusing on the clones after Order 66 that they slowly regretted their actions after their nightmares came to pass.
Sifo-Dyas never worked for the Sith or had anything to do with the Order 66 programming – it was only after his death that Darth Tyranus reached out to the Kaminoans and became the true leader responsible for the clones' secret chip.
Sifo-Dyas, a Jedi Master, ordered the creation of the Clone Troopers before the Jedi Council authorized it, and his story is explored in The Clone Wars. Sifo-Dyas' actions and his ties to Count Dooku had significant consequences, as his clone army ultimately served Palpatine's purposes and led to the Jedi's downfall.
The 7th Sky Corps was a corps of the Grand Army of the Republic under the joint command of Clone marshal commander Cody and High Jedi General Obi-Wan Kenobi.
'"The clones' nature posed a weakness, however,' Hux said. 'Because they were genetically identical, they were vulnerable to the same biological agents and pathogens—and, indeed, Separatist scientists spent much of the war seeking to create bespoke viruses that would target the clone armies.
Clones like Rex, Kix, Gregor, and ARC Troopers removed inhibitor chips and preserved their free will post-Order 66. The Bad Batch, Alpha-Class ARCs, and Cut Lawquane defied Order 66 due to immunity, free-thinking, or lack of contact.
The clone who took the name "Rex" was grown on Kamino like his fellow clone troopers, bred to be among the best soldiers in the galaxy. Rex received additional command training and earned the rank of captain, overseeing Torrent Company within the 501st Battalion.
Commander Cody was a marshal commander. They are the highest and most important clone commanders, having command over entire corps. Summary: Clones are cadets before completing the training. Clone privates and corporals are the lowest ranking soldiers and officers.
As a Sith Lord, Palpatine certainly understood the power of prejudice. The Clone Wars were designed to pit the Galactic Core - where humans were concentrated - against the more diverse Outer Rim.
Despite their intense indoctrination, enlisted stormtroopers were still capable of living civilian lives; they received pay, were given periods of leave time, and under some circumstances were given opportunities to retire from service.
With the clone army, Palpatine could convince the Senate that the Republic was capable of fighting a war and doing it without endangering citizens on the front lines. However, with the Empire in place, Palpatine needed enforcers who would encourage patriotic fanaticism more than he needed perfect soldiers.
