The history of the Legion can be traced back to the birth of the leader, Caesar himself, although the actual organization did not exist until several years later.
In 2226, a boy named Caesar was born. When he was only two years old, raiders killed his father. He and his mother survived, and the two of them fled the area and found respite with the Followers of the Apocalypse three years later. While living with the Followers, he eventually became a full member and received a good education, one that lead him to specialize in linguistics and anthropology (the study of the origins, development, characteristics, and social customs of humans).
When he was twenty years old, he was sent east with a man named Calhoun. Their mission was to study the different dialects of tribes with the help of a Mormon missionary named Joshua Graham. Caesar considered this to be a waste of time, but he went along with it anyway. The journey would change his life forever.
The expedition consisted of nine people. Together, they set off for what used to be Arizona before the Great War. Along the way, they found a sizable cache of books detailing the history of ancient Rome. Caesar read about Julius Caesar's accounts of his military activities in Commentarii and also read The History of the Decline and Fall of the Roman Empire. These books shaped Caesar into the man he became, and they acted as blueprints for his ideals and methods.
All was not well, however. In the year 2247, Caesar, Calhoun, and Graham were captured by the Blackfoot tribe and held for ransom. At the time, the Blackfoot tribe was engaged in a war with an astonishing number of seven tribes, and they were ill-prepared. Caesar, fearing that he would be killed if their captors came under attack, decided to train the Blackfoot tribe. He taught them how to maintain their guns and create explosives. He also educated them on the art of warfare, like small unit tactics and the idea of divide and conquer - striking at the weakest enemies first.
The Blackfoot tribe was impressed. In an ironic twist, they made Caesar their leader. Caesar started his conquest. The weakest opposing tribe, the Ridgers, was taken out first. The healthy survivors were taken as slaves, and Caesar ordered the rest to be murdered and had their mangled remains piled together. Next, he targeted the Kaibabs and destroyed them. Caesar and his tribe were successful mostly because he employed the concept of total war. Before he came to power, the tribes only battled each other in small skirmishes around the Wasteland. Caesar hated this idea and instead chose to defeat the enemies with total war - when a nation (in this case, a tribe) devotes every single one of their resources to defeating opponents.
The Legion is Formed
Caesar sought to absorb every tribe he came across into one, which he called the Legion. After the seven tribes were destroyed or incorporated into his own, he began traversing the Wasteland with Graham as his second-in-command for more tribes to absorb in a conquest that lasted for decades. By the 2270s, he had conquered 86 individual tribes and had effectively formed the largest and most feared organization east of the Colorado river.
The Battle of Hoover Dam
Eventually, Caesar's Legion moved to the west side of the Colorado river and constructed their base--Fortification Hill--in the Mojave Wasteland. Caesar then made the bold move to take on the New California Republic. He recognized the Hoover Dam as a strategic location, and in 2277 Legion forces under the command of Graham marched to the Dam. Initially, the Legion was successful in pushing NCR defenders back across the dam. At that point, the NCR Rangers along with the 1st Recon Battalion retreated west across the dam and entered Boulder City. The soldiers covered their line of retreat with explosives with the plan of trapping the Legion and wrecking havoc on them.
The trap worked. Graham was unable to adapt to the combat situation, and gave the fatal order to push towards the rangers. When the Legion offensive moved into Boulder City, the NCR detonated the explosives. The resulting blasts dealt a devastating blow to the Legion. The counter-attack by the NCR put an end to the battle as Legion forces fled.
The Second Battle of Hoover Dam
Near the end of Fallout: New Vegas, the President of the NCR, Aaron Kimball, arrives at the Hoover Dam to deliver a speech. At this time, Legion forces begin to secretly advance on the dam to launch a massive assault. During the speech, an agent of Caesar's legion tries to assassinate Kimball with a sniper rifle at long range. If the Courier is aligned with the Legion, they are the assassin. If they are aligned with the NCR, they must kill the assassin before they make the kill.
Regardless of whether or not Kimball survives, Caesar's Legion launches an assault on the dam, starting the Second Battle of Hoover Dam.
The Legion's military is organized by experience instead of rank. The recruits, who are the most inexperienced, are not given much armor and are the first to march into battle. The next "rank" up is the Decanus. These soldiers are the recruits who survived and proved their worth and battle, and they are placed in the middle. Finally, the last ones to go in are the veterans. Through this method, enemies that manage to survive the first two waves and usually weakened by the time they face their strongest foes in the back, putting them at a severe disadvantage.
Caesar's Legion mints their own currency, the Denarius and the Aureus. These types of currency hearken back to ancient Rome. The Denarius and Aureus are the most valuable currency in Fallout: New Vegas. The Denarius is equal to 4 caps, while the Aureus is equal to 100 caps.