Genetically engineered for strength and resolve, they are also short-lived and believe that "victory is life." They are bred to perceive the Founders, enigmatic shape-shifters who rule the massive Dominion, as gods, and are incapable of harming them. The Jem'Hadar's numbers are unknown, but they are produced by the thousands as needed.
Jem'Hadar are born in "birthing pods." As there is no individual reproduction, the Jem'Hadar are asexual (although all Jem'Hadar characters have been portrayed by male actors). After emerging from the birthing pod, the Jem'Hadar grows rapidly and is a battle-ready young adult in three days. They are bred without the need to eat, sleep, rest, relax or to pursue recreation, though during their growth periods they must consume massive quantities of food. The Jem'Hadar are also bred for proficiency in hand-to-hand combat, having an extremely tough and strong physique, with leathery, lizard-like skin. Jem'Hadar have a personal cloaking capability they call "shrouding". This ability appears to be a part of their integrated biology - in the DS9 episode "Hippocratic Oath", Jem'Hadar who are suffering from Ketracel White withdrawal lose their ability to shroud.
The only nourishment they require comes in the form of the hormonal enzyme Ketracel White (or simply "[the] white"). Without this drug, Jem'Hadar go into severe withdrawal, resulting in loss of cognitive and motor functions. They will lose their shrouding ability and regress into a highly aggressive stage, not discriminating in who or what they attack, including their Vorta superiors and peers. Lack of white will eventually kill a Jem'Hadar. The Founders use the Vorta to supply the Jem'Hadar with the white. It is administered through cylinders that fit into a system near a Jem'Hadar's collar, which is then fed directly into the Jem'Hadar via a tube running into the carotid artery.
Early in the Federation-Dominion war, unable to get backup from the Gamma Quadrant, the Founders in the Alpha Quadrant engineered the "Alpha" Jem'Hadar. The Alphas have a variation in the facial and head structures and more flippant attitude toward doctrine and procedure; the latter created animosity between the two breeds.
The Jem'Hadar are typically extremely loyal to their overseers. In "Rocks and Shoals", a group of Starfleet officers led by Captain Sisko are stranded on an uninhabited planet along with a group of Jem'Hadar and their Vorta, Keevan. Keevan is injured and needs medical attention to survive, so he decides to surrender himself. He informs Sisko how he will order his troops to attack so that Sisko can ambush and kill them. Even though the Jem'Hadar leader suspected Keevan's plan, so strong is their loyalty that they willingly walk into the trap as ordered. Earlier, in "The Ship", a group of Jem'Hadar commit suicide because they fail to save the life of a Founder.
However, other Jem'Hadar have been less devoted. "To the Death" featured a group that gained control of an Iconian gateway and plan to use it to rebel against the Founders. Their leader expected this group to pick up considerable support, to the point where the Jem'Hadar as a whole would be able to seize control of the Dominion and liberate themselves from the Founders. Even after these had been defeated, however, the loyal Jem'Hadar first murdered their Vorta for questioning their loyalty.
The Jem'Hadar command structure is fairly straightforward. A particular unit's leader is called a "first", the second in command is the "second", and so on down the chain of command. In "To the Death", the ranking Jem'Hadar gave the following battle dirge to his assembled troops:
(In one of the few joint Starfleet-Jem-Hadar missions, Miles O'Brien parrots this phrase, saying 'My name is Miles Edward O'Brien. I am very much alive, and I intend to stay that way.')
When someone questions an order of a superior Jem'Hadar, or even of the Vorta, the superior reminds them to the "order of things", by this phrase:
For which the answer is:
The Jem'Hadar also use edged weapons, including a small, curved blade designed to be held in one hand (and often used, rather oddly, as a bayonet on their energy carbines), and a larger sword-like weapon usually held with two hands. In early encounters, Jem'Hadar can walk through Starfleet forcefields. According to startrek.com, this ability is derived from energy-absorbing pads built into their uniforms. (It should be noted, however, that the same source also indicates their shrouding ability to be derived from technology, despite the fact that the episodes "The Abandoned" and "Rocks and Shoals" explicitly established that ability to be biological.)
The Jem'Hadar boast starships of unique design. These include relatively small, beetle-like attack ships that are comparable in size to the USS Defiant, larger battleships first seen in "Ties of Blood and Water", and rarely-seen behemoths first seen in "Valiant".
Their ships' bridges contain no central viewer, with only the First and their Vorta permitted to use a head-mounted display head-set to see outside the ship. There are no seats for the crew; everyone must stand (in the episode "One Little Ship", the First orders the chairs removed from Defiant's bridge). The beetle-like attack ships are sufficiently well-built to survive plummeting uncontrolled into a planet's surface and subsequently being repaired and flown again (DS9: "The Ship").
Hostilities between the powers of the Alpha Quadrant and the Dominion begin with the Jem'Hadar's kidnapping of Benjamin Sisko and Quark. Simultaneously, the Jem'Hadar destroy several Alpha Quadrant species' colonies and ships in the Gamma Quadrant. The Jem'Hadar kamikaze attack and destroy the USS Odyssey while the damaged starship is retreating after a mission to rescue Sisko and Quark. (DS9: "The Jem'Hadar")
The Jem'Hadar are the spearhead for the Dominion's invasion of and advances through the Alpha Quadrant. Although the Dominion allies itself with the Cardassians and the Breen, the Jem'Hadar remain at the forefront of the Dominion's forces. A Dominion reinforcement fleet would have spelled doom for the Alpha Quadrant defenders had the wormhole aliens not intervened and caused the fleet to vanish (DS9: "Sacrifice of Angels"). Even without these reinforcements, the war persists — and the Jem'Hadar continue to fight — for almost two more years.
After heavy losses, the Dominion War ends in 2375, and the Jem'Hadar withdraw to the Gamma Quadrant.