The following is based on Van Buren and has not been confirmed by canon sources.
A major fortification located in the American southwest, the Fort was destroyed by Caesar's Legion's forces, reducing the Republic's control over the frontier. The name Fort Abandon was applied to it after the NCR withdrew its forces from the Region.
↑ 1.01.1The Courier: "Why would I have brought it here?" Ulysses: "I've walked the East. You've walked the West, more than I have. Circle Junction. Reno. Vault City. Word of you at Fort Aradesh... Fort Abandon. Even further West than that, Brahmin drives on the Big Circle. Whatever you saw out there, wasn't enough to make you stay. Maybe the markings on the package reminded you of the road home." (Ulysses' dialogue)