One of the Enclave's Power Armor Mk. II-equipped Special Operations Squads, Leonidas was commanded by Colonel John and Lieutenant Chevy Bragg, and tasked with particularly sensitive operations deep in enemy territory, such as locating and conscripting surviving Prime Normal Vault populations to bolster the Enclave's dwindling numbers. The Braggs survived the destruction of Control Station Enclave by the Chosen One in 2242. After regrouping in Navarro, Leonidas participated in the NCR-Enclave War in northern California and southern Oregon. John was captured and sentence to an NCR prison, while Chevy pulled back the surviving members of Leonidas East, to the fringe of NCR territory, staying with Raider Alliance, with whom the Enclave had traded in exchange for Uranium and test subjects. Bragg would eventually escape NCR jurisdiction, reuniting with Chevy and receiving orders from surviving Enclave commanders to hunt down "Project Brazil," data on which of which was traced to Andrew Oran's personal Vault.
The Bragg's mission was to secure Project Brazil, indoctrinate surviving Vault residents as a new generation of untainted soldiers for the Enclave, and secure Vault 18 as a foothold from which the Enclave could strike back at the NCR. The Bragg siblings posed as wastelanders and secured the trust of the Wasteland Scouts by scavenging a Water Chip from Vault 16, and would spend 8 years living among the Vault populace and turning its young residents against the Vault leadership and to venerated the Enclave. Meanwhile Squad Leonidas waited in Athens-Tec Mine, as tensions grew with their Raider ''allies,'' particularly under the leadership of Juan Maxson-Elsdragon. When the Braggs received word that all surviving Enclave personnel were commanded to travel to the ruins of Washington D.C. by a mysterious ''President John Henry Eden,'' Leonidas took action, inciting a revolt among Bragg's Enclave Patriots while Squad Leonidas personnel waiting on the surface to breach the Vault and reunite with the Braggs.
Sensing weakness, the Raider Alliance struck, turning on their supposed allies, overwhelming the Enclave personnel and stealing their advanced weapons, Power Armor, and Vertibirds while the command staff was butchered or captured at Athens-Tec. With the conflict inside the Vault leading to the reactor entering meltdown, the surviving Vault residents, Patriots, Leonidas Squad and Survivalists all dispersed into the Pass. The Bragg twins, having escaped the Vault alive, would set out to regroup any surviving Enclave personnel in the Pass and strike at both the NCR and Raider Alliance.