The Federation Phalanx, also known as the Phalanx, was the name given to a region of disputed space on the Federation-Klingon border. The Phalanx was also bordered by several other races, including the Romulans, the Airee and the T'landra. (TOS novels: Enterprise: The First Adventure, The Children of Kings)
By the early 2250s, the Klingons maintained a weapons facility located on the planet Kitulba, at the base of the Phalanx. In 2251, the scientists on Kitulba were developing a cloaking device code-named "Black Snow Seven". (TOS novel: The Children of Kings)
By the mid-2260s, Starfleet operated thirty outposts and starbases in the region, with the primary base being Starbase 13. (TOS novel: Enterprise: The First Adventure)