The Barrier Alliance (sometimes the Orion Barrier Alliance) was a regional organization of so-called "neutral" star systems that was in existence until the late 23rd century. (Orion Press: "Incident on Xantharus", "The Salos Sellout", "The Wages of Vengeance", Ad Astra Per Aspera, "Return to Xantharus")
The Orions were the "guides" of the alliance, which included the Avians, Ursinoids, and the Catullans. While the Barrier Alliance ostensibly provided for mutual self-defense, regional cooperation, and peaceful settlement of controversies, it was actually a major source of slavery, illegal substances, and illegal weapons.
An early Earth warp-drive vessel, SS Defiant, one of two launched in 2064 to find the limits of human reach with an improved FTL system, was lost a year after its departure. It was later discovered that Defiant had been lost in an area of space claimed by the Barrier Alliance. (Star Fleet Starship Recognition Manual: Bonaventure Cruiser)
Captain Wolf Klawitter, aboard the USS Chickamauga, led a task force of starships to hold the line against an incursion of Barrier Alliance vessels into Federation territory. However, Starfleet Command adjudicated his response as excessive, and relieved him of command. (Starship Deimos)
- In the Orion Press continuity, the Barrier Alliance worlds joined the Federation in 2285. If that still holds for the fan films influenced by that continuity, then the incident occurred before that year.