Utterly ruthless and implacable, the Borg are a synthesis of flesh and machine fused by nanotechnology into an unstoppable force seeking the absolute goal of perfection. It is unknown how the Borg first came to be but common theories suggest that they began when a species experimenting with artificial intelligence and nanotechnology discovered to their horror that it had gotten beyond their control.

The Borg assimilate races and their technology adding their distinct uniqueness to their collective in order to draw closer to perfection. The Borg are aware of the value of diversity but extinguish it to enrich their own resources. Planets that the Borg encounter are often strip minded by them and turned into unlivable poisoned environments for anyone but Borg drones-- as happened to Earth in an alternate universe in which the Borg successfully prevented Earth from making contact with Vulcan. (See Star Trek: First Contact) Members of those races whom they successfully capture are transformed into Borg drones, their bodies infused with nanotechnology and their minds integrated into the Borg Collective which robs them of free will.

Starfleet's first encounter with the Borg came about when Q, a nearly omnipotent being of the mysterious Q Continium, transported the Enterprise into a region occupied by a Borg Cube. Though the U.S.S. Enterprise escaped with Q's help-- the Borg were now aware of humanity and made repeated efforts to annihilate it. In Best of Both Worlds, a single cube smashed through all of Earth's defenses, kidnapped Picard and transformed him into Locutus. A second assault took place when a Borg sphere traveled back in time to prevent first contact. In the prequel Enterprise, Borg who had been found under antarctic ice revived and came to life menacing the Enterprise.

A Vulcanoid splinter race of the Vulcans, the Romulans are xenophobic and conspiratorial. Robbed by totalitarian Preators and Senate of their former honor, treachery and deceit have become the watchwords on Romulus. Where the Tal'Shayar, the Romulan intelligence service spreads fear at home, the sleek green warbirds spread terror abroad. The Romulans who suffered Orion attack once are determined not to repeat the experience.

Romulans prefer to attack through stealth rather than openly, employing cloaking devices to conceal their ships and an extensive network of spies in Federation space, sometimes posing as the nearly physically identical Vulcans. While the Federation has briefly enjoyed peace with the Romulans, they are generally hostile and manipulative.

The Cardassians were once a peaceful and religious race but suffered from deprivation and hunger before the military took power. The Cardassian military militarized the state creating an empire that expanded across the galaxy seizing worlds and enslaving races for raw material. The Cardassian occupation of entire worlds bred resistance movements and eventually losses led to the withdrawal of troops.

Cardassians were trained in military discipline from a very young age, taught to obey and not to ask questions and to accept the decisions of the state. The Cardassian judiciary presumes guilt from the beginning of the trial and serves only as a forum for forcing a confession from the accused. Cardassia has suffered a number of coups and changes of government, one of which led to an alliance with the Dominion that resulted in the near destruction of Cardassia.

Cardassian relations with the Federation have been troubled by disputes over the Demilitarized Zone and succeeding battles there with Federation settlers.

Not a truly dangerous race but an extremely devious mercantile one, Ferengi will steal anything that isn't nailed down and then sell it back to you. Ferengi live by the Rules of Acquisition which govern their commercial transactions. The exchange of money and the accumulation of profit in the form of Gold Pressed Latinum are their key objectives in life.

Ferengi women have no rights and being denied clothing are forced to live naked. Ferengenar is itself an extremely wet world. Ferengi society is centered around The Tower of Commerce.

Ferengi first contact with the Federation was hostile both times with the destruction of the Stargazer, Captain Picard's first ship and a second hostile encounter aborted only by the presence of a superior race.

The original enemies of the Federation, Klingons are at times honorable warriors and at other times treacherous foes. Able to deal deceitfully and fight courageously, makes the Klingon race a dangerous enemy.

Klingon society prizes the warrior above all else and views it as the only worthwhile quality there is. Klingon culture displays contempt for weakness and rewards only strength in battle. Klingon theology holds that Klingons slew their own gods that created them, making themselves the only gods that matter. The Klingons also believe that the cowardly descend down to Stovo'kohr, the Klingon version of hell.

After a botched first encounter, the Klingon Empire became a key opponent of the Federation competing for planets and resources. War seemed imminent until the intervention of the Organians forced a truce. It was however a cold peace broken by individual acts of violence until Chanchellor Gorkon determined to sign a peace agreement with the Federation after the explosion of a Klingon moon severely damaged their world.

These Khitomer accords were breached when Sisko intervened on behalf of Cardassia during the Klingon invasion of Cardassia leading to war between the Federation and the Klingon Empire. This war was aborted on the exposure of General Martok as a changeling and with the Dominion invasion a treaty was signed again.