The Krenim Imperium was the government of the Krenim people and a military power located in spatial grid 005 in the Delta Quadrant, 65,000 light-years from Earth. They used chroniton-based weapons. It was along the USS Voyager's route in 2374. In an alternate timeline where the Krenim Imperium was almost as powerful as in the original timeline, the Imperium covered 849 worlds across 5,000 parsecs. (VOY episode: "Year of Hell"; ST reference: Star Charts)
The Imperium reached its peak in the mid-22nd century, controlling over 900 planets spread across 5,000 parsecs. However, by the 2170s the Krenim were losing the war with the Rilnar, unable to win with conventional weapons. Annorax, a Krenim temporal scientist from Kyana Prime, began research into temporal weapons and devised plans for a temporal weapon ship, but this never went beyond the theoretical stage.
By 2374, adversaries to the Krenim Imperium included the Alsuran Empire, the Garenor, Malkoth, Mawasi, Nihydron, Ram Izad, Rilnar and Zahl civilizations. First contact with the Federation occurred on March 16th, when the starship USS Voyager traveled through grid 005. The commandant of a Krenim warship asked them to avoid the area because it was disputed. (VOY episode: "Year of Hell")
Captain Kathryn Janeway heeded the Krenim's advice and Voyager changed direction to avoid Krenim space, knowing that not doing so would result in a "Year of Hell". (VOY episodes: "Year of Hell", "Before and After")
By the 25th century, the Imperium had resolved its disputes with the Zahl peacefully, and the two powers had been staunch allies for fifty years. (STO - Better with Friends mission: "Blockade Runner")
Sometime during or prior to 2410, the Krenim Imperium was defeated nearly and destroyed by the Vaadwaur Supremacy at the behest of the Iconians (who feared Krenim temporal technology), along with the B'omar Sovereignty. After the Imperium's fall, Krenim and B'omar refugees sought shelter among other powers throughout the Vyntadi sector block, including the Zahl. (STO - Delta Rising missions: "Mindscape", "What's Left Behind")
Later that year, the Alpha Quadrant Alliance made contact with a hidden Krenim holdout in the Kyana system and made a deal to join forces in the war against the Iconian Empire and their servitor races. (STO - The Iconian War mission: "Time in a Bottle")
The Krenim subsequently formed a new nation with their allies, the Nihydron and Zahl, the Krenim Coalition. The Coalition planned to weaponize time itself and shared its technology with the Alpha Quadrant Alliance. (STO video game: The Iconian War)
Year of Hell timelines
When the Krenim were unable to win the war against the Rilnar, temporal scientist Annorax of the Kyana Prime colony developed the Krenim temporal weapon ship, a mobile platform capable of erasing everything between single molecules to entire civilizations from history. Annorax eradicated the Rilnar civilization. The temporal weapon ship was protected from changes because it was out of phase with the space-time continuum. The eradication removed a vital antibody introduced by the Rilnar into the Krenim genome, causing the death of 15 million Krenim by a disease. (VOY episode: "Year of Hell"; ST reference: The Official Starships Collection)
Annorax created another timeline to reverse the damage. As a result, the Kyana Prime colony was accidentally removed from the timeline. The weapon ship conducted numerous temporal incursions but the colony failed to reappear. The incursions eradicated the Alsurans and Malkoths. The ship was in continuous operation for 200 years when Voyager entered spatial grid 005 in 2374. (VOY episode: "Year of Hell")
The timeline changed again when Annorax eradicated a Zahl outpost. In that timeline, the Zahl had defeated the Krenim Imperium a generation before, overcoming their temporal weaponry. The Krenim Imperium was a minor power only.
In a timeline where the Zahl had been eradicated and the Garenor were a major power, the Krenim Imperium spanned 200 star systems, over 900 planets and possessed thousands of starships, including numerous warships. The eradication of the Garenor civilization on May 20th, 2374, led to a reduction of the Krenim Imperium to a pre-warp state, with a small territory, few colonies if any, and weak Krenim patrol ships. Voyager, protected by temporal shields, became aware of the changes now and was assaulted by the Krenim temporal weapon ship but escaped.
The subsequent eradication of the Ram Izad civilization led to a 52% restoration of the Krenim Imperium. In this timeline, the Nihydron and Mawasi were opponents to the Imperium, and Voyager formed a task force of three Nihydron warships and two Mawasi cruisers on October 29th, finally engaging the Krenim weapon ship on November 29th. A final temporal incursion occurred when Voyager rammed into the weapon ship as it phased back into normal space-time, causing a temporal incursion within the ship. The weapon ship subsequently erased itself from time, and all of Annorax's alterations were reversed. In the reset timeline, the Krenim temporal weapon ship did not exist. (VOY episode: "Year of Hell")
In the prime universe year 2410, Pross, the leader of a group of Krenim survivors on Kyana Prime, stated that the prime universe version of the Imperium was aware of Annorax's attempts to avert Imperial downfall in the alternate timelines, and of his defeat by an unknown alliance. Given his defeat, they had been content until the Vaadwaur invasion to allow the Imperium to decline without further interference with the timeline. (STO - The Iconian War mission: "Time in a Bottle")
In another timeline, Kes jumped back in time from her moment of death in 2379, changing the timeline with each jump. In the period between 2373 and 2374, Voyager traversed the Krenim Imperium and suffered from continuous attacks by Krenim warships. The Krenim ceased their attacks after Voyager left the Imperium. Kes's temporal stabilization by the Voyager crew in 2373 resulted in the main timeline, where Voyager reached Krenim space later but avoided them and the "Year of Hell". (VOY episode: "Before and After")
The Krenim Imperium was located in spatial grid 005, 65,000 light-years from Earth, in the Delta Quadrant. It contained over 900 worlds in 200 star systems, spread across 5,000 parsecs. These worlds included the Krenim homeworld and the Kyana Prime colony. Krenim territorial claims were disputed by neighboring species. In various timelines, the Krenim bordered several interstellar powers, including the Alsuran Empire, the Garenor, Malkoth, Mawasi, Nihydron, Ram Izad, Rilnar and Zahl civilizations. (VOY episode: "Year of Hell")