The Breen were a reclusive, powerful, and warlike humanoid race, native to the planet Breen in the Alpha Quadrant. Shrouded in mystery, the Breen were also one of the most underestimated races inhabiting that quadrant of space.

Historically, the Klingons were among the first to discover that the Breen didn't tolerate incursions into their space. During the Klingon Second Empire, Chancellor Mow'ga ordered an entire fleet of Klingon warships to invade and conquer the Breen homeworld. The fleet never returned and was never heard from again.

Breen privateers during the 24th century often conducted indiscriminate raids against other Alpha Quadrant species. This included the Breen attack and capture of a Cardassian transport, the Ravinok, in 2366. The survivors of that attack were used for slave labor in Breen-operated dilithium mines on Dozaria.

Even by the 24th century, though, much was still unknown about the Breen and their government, known as the Breen Confederacy. The Federation had limited knowledge of the Breen, however, and was aware of Breen outposts located in Sector 97 in 2368. That same year, Starfleet feared the Breen might have attacked the science vessel SS Vico there.