Klingon Empire

The Klingon Empire was the official state of the Klingon people. It was founded in the 9th century by Kahless the Unforgettable, who first united the Klingon homeworld of Qo'noS. Since then, the Klingon Empire expanded its sphere of influence by conquering numerous systems and incorporating them. This made the Empire a major power in its region of the galaxy as of the 22nd, 23rd, and 24th centuries.#

History

The mytho-historical origin of the Klingon Empire revolves around Kahless the Unforgettable, who emerged as a champion of the people in the 9th century AD and became the first Emperor. In the 14th century, the Empire faced one of its biggest threats when a race from the Gamma Quadrant the Klingons called Hur'q invaded Qo'noS. They looted many valuable cultural treasures, including the revered Sword of Kahless, but then left again. In the mid-21st century, the reign of the last Emperor ended and authority passed to the Chancellor of the Klingon High Council. (e.g., TNG: "Rightful Heir"; DS9: "Looking for par'Mach in All the Wrong Places", "The Sword of Kahless")
Because of a Human-caused medical crisis and later territorial conflicts, United Earth and subsequently the United Federation of Planets became the Empire's main adversary in the 22nd and 23rd centuries, during which cold as well as hot wars were fought. While the Empire was in disarray as of the first half of the 23rd century, the conflict with the Federation helped to unify the disparate Klingon Houses and stabilized the rule of the Chancellor. (e.g. ENT: "Affliction", "Divergence"; DIS: "Battle at the Binary Stars", "Point of Light"; TOS: "Errand of Mercy")
After the Praxis disaster of 2293, Chancellor Gorkon pursued peace with the Federation, resulting in the Khitomer Accords and ultimately paving the way for the Treaty of Alliance between both powers. Klingon-Federation cooperation persisted despite interference by the Romulan Star Empire and a Klingon Civil War from 2367 to 2368. (e.g. Star Trek VI: The Undiscovered Country; TNG: "Reunion", "Redemption", "Redemption II")
In the 2370s, the Dominion, a hegemonic empire from the Gamma Quadrant managed to sow discord among Alpha and Beta Quadrant powers, resulting in yet another brief Federation-Klingon war. Eventually, both powers joined forces and, after a costly war, together defeated the Dominion in 2375. (e.g. DS9: "Apocalypse Rising", "By Inferno's Light", "What You Leave Behind")

Government

Officially, the Klingon Empire was a feudal monarchy, with power residing in the Emperor, who was traditionally a descendant of Kahless. However, de facto power lay with the Klingon High Council. The position of emperor was abandoned (but not officially abolished) in the mid-21st century, but was revived in 2369 when a group of clerics created a clone of Kahless, who was accepted as the new Emperor, albeit only as a religious figurehead. (TNG: "Rightful Heir")
The Chancellor, the de facto leader of the Empire, was head of the High Council, which consisted of twenty-four members representing various Great Houses (tuqmey, essentially, the nobility). The Chancellor was protected at all times by the Yan-Isleth (Brotherhood of the Sword). (DS9: "Apocalypse Rising")
Women were not normally permitted to hold seats on the High Council. Despite that, Gowron once offered Ambassador K'Ehleyr a seat on the Council in exchange for her support of his bid to be Chancellor. (TNG: "Redemption", "Reunion") Also, Azetbur, the daughter of Chancellor Gorkon, was permitted to succeed him as Chancellor in 2293. (Star Trek VI: The Undiscovered Country)
It is possible that this was permitted because she was made the head of her house, due to the unusual circumstances of her father's death. The novelization of Star Trek VI: The Undiscovered Country indicates that Gorkon suspected that something might happen to him on the voyage and had arranged with his allies on the High Council for them to back Azetbur as Chancellor if he were to be killed. The novel Serpents Among the Ruins establishes Azetbur's assassination by a Klingon named Ditagh (β), who hoped that after her death the more powerful General Kaarg would rise to power. Kaarg would indeed rise to power and it would be him that would forbid women ever on the High Council again. Various factions almost constantly challenged the leadership of the Empire, and so over time the Klingons developed a strict and rigorous Rite of Succession to determine their leader. According to tradition, one was permitted to challenge the leader on the grounds of cowardice or dishonorable conduct and fight in single combat. Should the challenger slay the incumbent, he assumed the role of the new leader. (DS9: "Tacking Into the Wind")
Because of the Klingon propensity for violence, shrewd Klingon chancellors redirected hostilities outward, where they would otherwise cause a civil war. In the 2150s, the Klingon chancellor instructed Duras to recapture Jonathan Archer after the latter escaped imprisonment on Rura Penthe. In this way, the chancellor focused the blame for certain internal problems on an external cause. (ENT: "The Expanse") Likewise, Gowron focused his soldiers' energies on invading first the Cardassian Union and later the Federation in order to avoid internal conflicts at home. (DS9: "The Way of the Warrior", "Apocalypse Rising")
Aside from challenges to the primary leadership of the Empire, there was also frequent feuding between the various Great Houses. Most often, the challenge was made on the floor of the High Council and resolved on the battlefield. However, on occasion, some "dishonorable" House leaders chose to make more insidious attacks by undermining the standing of their enemies. D'Ghor underhandedly attacked the House of Kozak in this way in the early 2370s. (DS9: "The House of Quark")

Expansion

The Empire contained Klingon colonies as well as annexed, previously inhabited worlds. (e.g., ENT: "Judgment"; TNG: "The Mind's Eye") The foundation of a Klingon colony could be preceded by bio-surveying potentially suitable planets. (DS9: "Dramatis Personae") For conquered inhabited worlds, the traditional Klingon practice called for the execution of all government officials, and the installation of an Imperial Overseer, e.g., a governor, to put down any further resistance. An alternative approach was the creation of a puppet regime under a native governor. (TOS: "Errand of Mercy", "A Private Little War"; DS9: "The Way of the Warrior")
The planet Krios was a Klingon colony on the border of Federation space. In 2367, the Kriosians sought independence from the Empire. According to Klingon Ambassador Kell, "There was a time when the Empire would crush a rebellion. Now it is tolerated. We have enough problems on the home planet. We don't wish to divert resources to such a trivial war." When asked if the Empire considered granting them independence, Kell responded, "Perhaps. We'll conquer them again later, if we wish." (TNG: "The Mind's Eye")
Klingons were a very territorial species, vigorously patrolling the borders of their Empire with severe consequences for any trespassers. The Klingon Empire shared borders with the Romulan Star Empire and the Federation, the latter of which was protected by a neutral zone during the later 23rd century. (e.g. ENT: "Bounty"; Star Trek II: The Wrath of Khan; TNG: "Aquiel"; Star Trek)
In 2016, a Vulcan ship crossed into Klingon space near H'atoria and was immediately destroyed by the Klingons. (DIS: "The Vulcan Hello")
In 2152, the IKS Bortas tracked a refugee ship from the Klingon protectorate of Raatooras to a system just outside of Klingon territory, where they discovered that it had been rescued by the Human vessel Enterprise. Following an engagement between the two vessels, Enterprise Captain Jonathan Archer was put on trial on Narendra III, under the suspicion of conspiring against the Klingon Empire. While awaiting sentence, Archer was relying on Sub-commander T'Pol and Commander Charles Tucker III to get Enterprise safely out of Klingon territory whatever the outcome. Archer was sentenced to lifelong exile on Rura Penthe. As Enterprise was deep inside Klingon territory, T'Pol noted that an attempt to rescue the Captain was out of the question. However, she was able to recover Archer using diplomatic channels. (ENT: "Judgment")
The Tezra, a ship of the Tellarite Mining Consortium, was seized by the Klingons after Captain Skalaar crossed into Klingon territory while hauling firesalt. As Skalaar recalled, he hoped to trim two or three days off his journey by cutting across the most insignificant corner of Klingon space, only to learn very quickly that there was no thing as an insignificant corner of Klingon space. To pay his impound fee, Skalaar become a bounty hunter. In March 2153, he captured Jonathan Archer, intending to deliver him to the Klingons. Following Archer's abduction, Enterprise learnt from Kago that he was halfway through Klingon space. Less than six light years from Klingon space, Enterprise set a course to search and ultimately recovered their captain. (ENT: "Bounty")
In May of 2154, a group of Augments crossed into Klingon space aboard a highjacked Bird-of-Prey. Arik Soong suggested the Briar Patch as a hiding place once they were safely through Klingon space but his suggestion was rejected by Malik, the self-appointed leader of the Augments. Later, Enterprise used a Klingon warp signature and a universal translator programmed with seven Klingon language dialects to pass as a Klingon vessel within Klingon territory to intercept the Augments. (ENT: "The Augments")
Later that year, Enterprise set course for Klingon territory after Doctor Phlox was abducted. Captain Archer requested the assistance of Columbia in their search, noting in his starlog "We'll have a better chance of making it through Klingon territory with twice the firepower." (ENT: "Affliction", "Divergence")
In 2233 of the alternate reality, the USS Kelvin was on the edge of Klingon space when it was attacked by the Narada, a Romulan vessel from 2387 of the prime timeline. (Star Trek)
In 2256, during the Battle of the Binary Stars, T'Kuvma broadcast a message from his flagship on all subspace bands: "Members of the Federation. What you call your most remote borders, I call too close to Klingon territory." (DIS: "Battle at the Binary Stars")
Six months after the start of the war, the USS Discovery was running drills near Klingon space. It was later responsible for scuttling the USS Glenn, a sister ship which was adrift by Klingon territory. (DIS: "Context Is for Kings")
When Captain Gabriel Lorca was abducted in a shuttlecraft by a D7-class vessel, Acting captain Saru ordered Commander Airiam to inform Lieutenant Paul Stamets that Discovery may be making multiple spore jumps in rapid succession throughout Klingon space in an attempt to track Lorca's captors. Repeated use of the spore drive in attempts to rescue Lorca resulted in Ripper entering an extreme state of cryptobiosis and becoming unresponsive. Stamets resorted to injecting himself with a sample of tardigrade DNA and entering the spore chamber to act as the navigation computer. The jump was a success and Lorca was recovered, along with another Starfleet prisoner, Lieutenant Ash Tyler, with Discovery departing Klingon space. (DIS: "Choose Your Pain")
In 2259, Khan Noonien Singh took refuge in the Ketha Province on Qo'noS, knowing that for Starfleet to approach Klingon space would risk a war. After using a K'normian trading ship to recover Khan from Qo'noS against the orders of Admiral Alexander Marcus, Captain James T. Kirk found that a warp core malfunction had stranded his ship on the edge of Klingon space. As it transpired, Marcus had the Enterprise warp core sabotaged, intending for the Enterprise to use advanced long-range torpedos against Khan on Qo'noS. The plan was for the Klingons to discover the Enterprise and in response to the torpedo attack, start a war with the Federation. Thankfully, Pavel Chekov was able to repair the warp drive, allowing the Enterprise to return to Federation space. (Star Trek Into Darkness)
In 2293, the USS Enterprise-A entered Klingon space, masquerading as the freighter Ursva, to rescue Captain James T. Kirk and Doctor Leonard McCoy from Rura Penthe. Meanwhile, Captain Hikaru Sulu ordered the USS Excelsior on an aborted raid into Klingon territory from Sector 21185 on an ill-advised rescue attempt. Sulu, who attempted to sneak the Excelsior into Klingon territory by traversing the Azure Nebula at maximum warp, was discovered by a Klingon battle cruiser commanded by Kang. While Sulu explained that the Excelsior was on a survey mission, studying the nebula, and had gotten lost due to a malfunction in the navigation system, Kang offered to escort the Excelsior back to Federation space. As the Excelsior emerged, she fired a modulated positron beam that triggered a thermochemical reaction, and successfully disabled Kang's ship. The Excelsior then made a break for Qo'noS, but was almost immediately intercepted by three Klingon battle cruisers. The Excelsior and Klingon ships continued to exchange fire until the Excelsior was forced to retreat back to Federation space. (Star Trek VI: The Undiscovered Country; VOY: "Flashback")
Following the outbreak of civil war in the Empire in late 2367, Captain Jean-Luc Picard of the USS Enterprise-D led a Federation fleet into Klingon territory in order to prevent Romulan forces from covertly supplying the House of Duras. Though Commander Sela noted that Romulan vessels could slip through Klingon territory at will via cloak, Picard employed a tachyon detection grid to detect them. (TNG: "Redemption II")
In 2369, the Enterprise found Relay Station 47 abandoned, with evidence suggesting that encrypted messages sent from there had been tampered with. As the station was close to Klingon space, the Klingons came under suspicion in connection to the disappearance of the crew. The lone survivor, Aquiel Uhnari, took the station's shuttlecraft Verne, which was later found by Klingon patrol ship on the Klingon side of the border, on a course for Sector 2520. According to Governor Vagh, who was in charge of that regions.
In 2372, during the Klingon-Cardassian War, the Pentath system bordered Klingon territory. When an outbreak of Rudellian plague struck the Cardassian colony on Pentath III, the Cardassians feared raids and so requested protection from Starfleet for their convoys of medical supplies and relief workers. (DS9: "Rules of Engagement")
In 2373, a memory virus that Lieutenant Commander Tuvok had contracted aboard the Excelsior eighty years earlier led him to advise extreme caution when close to Klingon space. Now aboard the USS Voyager, Tuvok was reminded by Lieutenant B'Elanna Torres that the Klingon Empire was on the other side of the galaxy. (VOY: "Flashback")
During the Federation-Klingon War, Starfleet sent an infiltration team to Klingon territory in order to prove that Klingon Chancellor Gowron was a shapeshifter. This team consisted of Captain Benjamin Sisko, Lieutenant Commander Worf, Constable Odo and Chief Miles O'Brien. Their mission was complicated when they learnt from Starfleet Intelligence that Gowron had relocated Klingon military headquarters to Ty'Gokor. Located in an asteroid field deep in Klingon space, Ty'Gokor was considered the heavily fortified installation in the Empire. (DS9: "Apocalypse Rising")
In an unknown, future timeframe, the USS Pasteur crossed the border into Klingon territory to scan the Devron system for a temporal anomaly. (TNG: "All Good Things...")