The Grand Army of the Republic (GAR), is a legion or army of Clone Troopers and Jedi under the command of Chancellor Sidious Palpatine (Darth Sidious).
HistoryThe legion's history first began when Jedi Master Syfo-Dias secretly experimented with clones on the planet or world of Kamino. To where he tested different species of aliens, creatures, and even himself to create a massive army for the Republic.
Syfo-Dias wanted to create the army, because of the fact that the Republic began to become overrun by the Confederacy of Independent Systems (CIS) or simply the Seperatists. But his plan unfortunately failed, when the merciless and corrupt Mandalorian bounty hunter known as Jango Fett hired by Darth Tyranus (Count Dooku) of the Seperatist Army, assassinated him, and tested his genes on Kamino, which were a huge success.
Jango Fett actually wanted the once-secret Clone Army for himself, to whom he wanted to have his own version of the Mandalorian Death Watch militia, and even his own personalized Cloned son known as Boba Fett.It wasn't until Obi-Wan Kenobi tracked-down Jango Fett to Kamino, to where he learned all about the Clone Army, and wanted to prevent the army for being part of the Seperatists.
Thus, Jango meeting his fate, and the army being used to create the legendary legion known as the Grand Army of the Republic.
After spending roughly four years of fighting during the ruthless and violent Clone Wars Era, the galaxy was revealed to Darth Sidious, who disguised himself as Chancellor of the Republic the entire time. And thus, him corrupting and conquering the galaxy by seducing Jedi General Anakin Skywalker to the Dark Side of the Force, and having the Clones execute Order 66; an order to eliminate the Jedi for betrayal; had the Clones, Jedi, and Anakin himself, unfortunately tricked.The GAR eventually became the Galactic Empire, 501st Legion "Vader's Fist", or simply the Empire.
The Clone Troopers and the Grand Army of the Republic are considered to be the most confusing topics in the Star Wars franchise.
Most fans often confuse the 501st Legion actually being the entire Grand Army of the Republic, yet the entire army was absorbed into what was left of Anakin Skywalker's original battalion of Clone Troopers, known as the 501st Torrent Co.