The Empire was founded by Kahless the Unforgettable after he united the Klingon homeworld of Qo'noS in the 9th century. The Empire expanded beyond its homeworld though colonization and military conquest to become a major power in its region.
During the 23rd century, the Klingon Empire and the United Federation of Planets had a extended period of cold war. This erupted into a brief war in 2267, which was stopped by the intervention of the Organians. (TOS: "Errand of Mercy")
- In other continuities, the so-called cold war was not nearly as cold. The FASA RPG posited a Four Years War years before the encounter with the Organians. Many continuities include that war, though they place it at different times. In the continuity of Absolute Horizon, such a conflict broke out c. 2255 though it ended with a cease fire two years later.
In 2293, the accidental destruction of the moon Praxis and contamination of Qo'noS saw Klingon Chancellor Gorkon seek a controversial peace treaty with the Federation. Despite efforts on both side to derail the process, which included the assassination of Gorkon and attempted assassination of the Federation President, the historic Khitomer Accords were signed formalising peace between the Klingons and the Federation. (Star Trek VI: The Undiscovered Country)
In 2372, the Klingon Empire went to war with the Cardassian Union after they came to believe that Changelings were responsible for the overthrow of the military government. The Federation Council denounced the invasion and in response Chancellor Gowron left the Khitomer Accords, ending their alliance with the Federation.
After the USS Defiant rescued the Union's civilian goverment and exchanged fire with Klingon warships and the Klingon fleet attacked the Federation-administered Deep Space 9 the Empire and Federation came to the brink of war. It was only averted when Gowron disengaged from the assault when faced with Starfleet reinforcements. (DS9: "The Way of the Warrior")
In late-2372, the Klingons demanded the Federation leave the Archanis sector, despite ceding it to the UFP a century before. When the Federation refused the demands the Klingons declared war on the Federation. After three months General Martok was exposed as a Changeling who had been manipulating the Klingon Empire and a ceasefire was agreed to between the warring parties. (DS9: "Broken Link", "Apocalypse Rising")
In mid-2373, the Cardassian Union joined the Dominion and received a large reinforcement of Jem'Hadar warships and troops. They were able to drive the Klingon Empire from its Cardassian territories. Faced with this new threat Gowron reentered into the alliance with the Federation so to oppose the Dominion. (DS9: "By Inferno's Light")
In 2401, the Klingon Empire was at war against the Lyran Star Empire. (Star Trek: The Stoneship Files: "At the Soapbox Races") Peace was reached and the Klingon Empire has also lowered tuition in late 2401. (Star Trek: The Stoneship Files: "The Defector")
The Klingon Empire was a feudal monarchy with an Emperor at its head, with the first been Kahless. By the mid-21st century no emperor sat on the throne and the Klingon High Council ran the Empire. In 2369, a clone of Kahless was created and he took the throne largely as a religious figurehead. (TNG: "Rightful Heir")