The Pilmarian Union of Planets, collectively known as the Pilmarians, was a conglomeration of planets joined together in one political entity.
Founded in 2208 by 12 planets, the Pilmarian Union grew to 87 members by 2374. One of the founding planets, Butal Prime, was comprised of a millenia-old monarchy. All the founding races of the union created the capital on an unsettled Class M planet in the Butal system, naming it Pilmaria.
The relationship between the Pilmarian Union and Steranmio Alliance was always strained. When Demanal IV and several other planets seceded from the union in 2271 to form the Steranmio Alliance, the remaining members of the union suffered severe economic and political problems. A cold war between the two powers started, going so far as to damage members of the same race on either side of the zones between them.
As the cold war stewed, hatred between the two sides grew until, in 2374, the tensions exploded into open warfare. Thousands of refugees tried to flee to non-aligned worlds, but the war quickly spread. As the Hanej Gateway opened and the refugees flowed through, the war growing more intense and bloody, members from the Pilmarians, the Steranmio and non-aligned worlds sought the help of the United Federation of Planets to negotiate a peace. A Starfleet task force, led by the USS McCaffrey and accompanied by the USS Cantabrian-A and USS Prospect-A, arrived in late 2374 to assist in the process and keep the peace. (Star Trek: The Cantabrian Expeditions: "Said I Was Saved but You Never Said What From")
The Pilmarian Union of Planets spanned nearly 2,000 light years and was approximately 7,000 light years from the Federation. Situated between the Pilmarians and Federation was assumed to be Renai'i territory. Without the Hanej Gateway, a nearly instantaneous journey between the two areas, it would take an average starship seven years to get between the two areas at a high rate of warp. (Star Trek: The Cantabrian Expeditions: "Said I Was Saved but You Never Said What From")