The 501st Legion (also known as "Vader's Fist") is a fictional Clone Trooper (and later Stormtrooper) unit from the Star Wars movies and Expanded Universe. They served under Darth Vader, and were considered to be the best soldiers in the galaxy. Most of the story linked to the 501st comes from the game Star Wars: Battlefront II.
The Legion's name is based on a fan organization of the same name; their inclusion in the official continuity was based on the worldwide organization's dedication to Star Wars fandom.
The 501st Legion, like all Clone Troopers and Clone Trooper Legions, was created on Kamino; cloned from the genetic samples provided by famed bounty hunter, Jango Fett, and bred to serve as part of the Grand Army of the Republic. They served in the first actions of the Clone War, and assisted in the assault on the Jedi Temple. They appeared identical to the 1.25 million clone units created by the Kaminoans. However, the actions of the 501st, as well as the 65th Homeworld Security Shocktrooper Legion under the command of Thire, during the Battle of Geonosis became the center of special interest from the Supreme Chancellor Palpatine. During the Clone Wars one team of Clones from the 501st were lead by Captain Rex ( an ARC Trooper captain who leads the 501st and reports directly to General Skywalker).
Both Legions were transferred to a secret facility on Coruscant, where they were retrained in advanced infantry, commando, and space-combat techniques. The 501st Legion was kept on Coruscant until the Battle of Jabiim highlighted the level of threat the CIS posed to the Galactic Republic. In response to this, Palpatine authorized the 'release' of members and squads of the 501st to other Clone forces, in order to spread their specialist skills amongst the fronts of the Clone Wars. These clones were absorbed into their new Legions, but a record of each clone's location was kept in secret, so that the entire Legion could be pulled back together when necessary. Some clones were sent in small detachments to serve in space battles.
During the final weeks of the Clone Wars, members of the 501st were being continually deployed between fronts. They were split into small teams, sometimes used to supplement other units of the Republic Army, at other times operating independently of the existing forces and command structure of the warzone. Members of the 501st Legion served at Felucia, Mygeeto, Kashyyyk, and Utapau during the last days of the Clone Wars. The 501st did not specialize in any particular combat skill, and had members who served in roles such as combat engineers, anti-tank infantry men, and fighter pilots.
Although a substantial number of the Legion was offworld at the time of the Battle of Coruscant, the forces remaining on Coruscant were instrumental in the world's defense by weakening the defenses of Grevious's flagship, which would make the Jedi raid on it easier. Under the leadership of Commander Appo and the newly turned Darth Vader, the 501st was instrumental in carrying out the first action of the Great Jedi Purge: the sterilization of the Jedi Order. Journal entries made by members of the 501st prior to this event suggest they knew that they would have to kill the Jedi, but ultimately would follow the Emperor's orders. This is confirmed by the retired trooper in the beginning of the mission "Knightfall" in Star Wars Battlefront II. Saying that they "said nothing and may have private traitorous thoughts", this showed that the troopers may have had some form of compassion, but that their orders overrode this.
The 501st was assigned by Palpatine to act as the personal soldiers of Darth Vader. During the early years of the First Galactic Empire, the Legion saw service on many worlds, putting down rebellions on Naboo and Kamino, and prevented the efforts of Gizor Dellso to reignite the Clone Wars. The aforementioned rebellion on Kamino saw the 501st pitted against fellow clones as a revolt staged by their creators. After the conflict's end, it was considered by the Empire that replicant soldiers were too susceptible to organize an insurrection, and therefore continued production of clones based on varied templates other than just Jango Fett. During this time, the Legion came to be known by its nickname - "Vader's Fist", and became the only legion to be completely made up of Jango Fett clones. The Legion was later assigned to the First Death Star as a security force; the majority of the Legion survived the Battle of Yavin as they were 'off-world' at the time, attempting to track down the stolen technical readouts.
The 501st Legion continued to serve the Empire throughout the Galactic Civil War, including playing a key role in the Empire's victory at Hoth. Little of their history is known after Hoth, although after Vader's death at the Battle of Endor, the Legion was dissolved and its members were requisitioned by Imperial ex-officers during the internecine fighting that followed.
The victory proved to be short lived, for with the destruction of the second Death Star, the empire was left in shambles, and the 501st was destroyed by the warring remnant factions. However, the 501st were to be resurrected in name through Grand Admiral Thrawn.
An armed force under the name of the 501st Legion did not come back into being until the designation was resurrected by Grand Admiral Thrawn. Thrawn's 501st was an elite unit charged with the defense of the Hand of the Empire (also called as the Empire of the Hand in some novels), however it was no longer completely composed of Jango Fett clones. It should also be noted that by this time the 501st was composed of aliens as well as humans, an incredible thought considering how xenophobic the Empire under Palpatine was.
This is a list of major and minor conflicts the 501st were involved in.