The Core Region is a portion of the Fallout World that corresponds to the twentieth century’s American West Coast and southwest in general. It was the focus of the Fallout games prior to Fallout 3 and, as such, has much more detail to it than other regions of Fallout Earth. It is the setting with which fans of the games are most familiar; being the first Wasteland introduced and also the largest one.
The basic Core Region stretches from Baja in the South to Klamath in the North. It covers most of California, as well as parts of Oregon, Nevada and North Mexico. It was a largely inhospitable region, which is why most people chose to seek refuge in the remains of old cities, where they found (relative) safety in numbers. However, as resources grew more and more scarce, people and creatures came to consolidate into groups of their own kind, so locations are far more homogeneous than one might expect from a world ravaged by mutation. By the time the N.C.R dominated all of the Core Region, except for South Oregon, peace came between humanoid creatures.
The ghoul population of the wasteland was mostly clustered around the pre-War city of Bakersfield, California (now known as Necropolis), but were scattered after an attack by the Master's army. Most of the surviving ghouls eventually found a home in Gecko or Broken Hills along with humans and super mutants, both far to the north-east of their former home.
During the events of the original Fallout, the only faction in the Core Region was the Brotherhood of Steel. The Brotherhood was very secrative and did not conquer or seek to unify anything, they just stayed alone, although their prescence was known and they were feared throughout the Core Region. As the Brotherhood re-sparked old sciences in secret, humans and ghouls outside the wastes seperated themselves, tending to live far away from each other. This made time for development by both factions. Humans set up many cities on the ruins of old cities and the ghouls stayed close to eachother living in very few cities. Many cities, such as the Hub and L.A, grew large very quickly and accepted humans and ghouls living with each other, the population was a human majority though. Around this time, the Master became the abomination that he was.
The Mariposa Super Mutants
The Vault Dweller from V13, then saved Shady Sands from the Khans, a group of raiders. The Dweller then went on to do many great deeds in South California that affected the rest of the Core Region and even the Mojave Wasteland. The Dweller killed the Master who was planning on using his Super Mutants to take over the America's and possibly the Eastern Hemisphere, if he could reach it. His Super Mutants were created using the F.E.V (Forced Evolutionary Virus), this could turn Ghouls and Humans both into Super Mutants. Before their defeat, the Super Mutants ravaged the Ghoul city of Necropolis, leaving the Ghouls scattered. The Super Mutants, with their link to the Master gone, spread throughout the wastes, forming many societies. The Dweller was then exiled from V13 and with other Vault Dwellers went on Northwards to South Oregon and formed the Tribal Village of Arroyo.
If Dweller had not saved Shady Sands, the small community would not have grown into the bustling faction: The New California Republic. The N.C.R, however, treated Dweller as nothing more than a legend. When the N.C.R unified South California, they brought peace amongst the Ghouls and Humans who then began to live in peace together, although discrimination is unavoidable. Around this time, the Brotherhood's presence began to wane and eventually, for a while, they became a myth and were not known to any other than the N.C.R. The N.C.R then moved on northwards in the Core Region. The Enclave, who were experimenting with the F.E.V, were about to spread it all over the world (an effect that would kill anyone with any trace of mutation) until they were stopped by the Chosen One. The Chosen One was the grandson of Dweller and had come to save the near dead village of Arroyo. The citizens of Arroyo had been kidnapped by the Enclave with the V13 inhabitants. With the G.E.C.K, the two communities were reunified under New Arroyo which then became a bustling location. Navarro, the operating base of the Enclave, was then destroyed by the N.C.R who claimed the land and shoved the Enclave out to the East Coast. Following the destruction of the Enclave, the NCR annexed many territories including Klamath and New Arroyo, they also continued their spread into North Nevada, part of it which is in the Core Region.
Unification of the Core Region
As the N.C.R expanded into the Mojave Wasteland, the Brotherhood was once again awakened. They proved to everybody that they were more than just a myth by trying to strangle technology out of the hands of whom they considered lesser beings, most notably the N.C.R who had finished their conquest of California. The N.C.R always had a numerical advantage over the Brotherhood. The Brotherhood was then diminished from the Core Region and was only active in the Mojave Wasteland, in the West Coast, while the N.C.R was sent to near bankruptcy and could operate only on its funds from Helios 1 and Hoover Dam. For this they looked to aquire New Vegas, something the Caesar's Legion also hoped to do. Possibly, in hopes to aquire more money the N.C.R annexed Baja and Baja Sur, thus gaining control of all the Core Region. While the N.C.R has stabilized the Core Region, it is unknown what affects the events at the Mojave Wasteland will have on the Core Region citizens, a change in taxes is for sure though.
There are very few factions in the Core Region that aren't limited to one or two towns. After the Master's defeat, the New California Republic became the most influential organization that unites several major towns, including Shady Sands (its capital, now known mostly simply as NCR), the Hub, Junktown, Maxson and Dayglow. The major goal of the NCR was to unite all of California under a single government, a goal that was accomplished by the time of New Vegas.
In contrast, the Brotherhood of Steel is a mysterious techno-religious organization, with roots in the US military. While few in number, they make up for this with their arsenal of pre- and post-War technology: They have laser weapons, Power Armor, surgical enhancements and combat implants. Unlike the NCR, the Brotherhood is not interested in conquest nor in saving humanity, but instead in maintaining their secrecy as well as preserving and developing technology. However, by the time of New Vegas, the only Brotherhood chapter on the West Coast exists in the Mojave Wasteland and not the Core Region.
The Mojave Outpost
A small portion of the Core Region appears in Fallout: New Vegas, from the California-Nevada state line to the edge of the map, encompassing the region near Nipton and the covers the Mojave Outpost. The Mojave Outpost, however, is not part of the Core Region or the Mojave Wasteland but rather a boundary or border between the two wastes.