(covers information from several alternate timelines)
The Krenim Imperium were a military power in the Delta Quadrant. Located in spatial grid 005, the Imperium's weaponry was chroniton-based, making them a formidable foe, as their torpedoes existed in a state of temporal flux, allowing them to penetrate deflector shields with ease.
The Imperium's temporal science had advanced to such a state that by the 2170s they were able to develop a weapon-ship, capable of erasing a target from the space-time continuum. This ship was used to eliminate the Rilnar, who were the greatest enemies of the Imperium. However, the removal of the Rilnar led to a plague that almost destroyed the Krenim completely. In a bid to restore his lost race, the vessel's commander, Annorax, removed countless civilizations from history. After 200 years of trying, the Imperium was still a shadow of its former self and had been taken over by the Zahl.
In the 'original' timeline for Voyager, Krenim ships wandered Zahl space claiming it as their own, but lacked the military might to enforce their claim. Following the erasure of the Zahl from time, the Imperium became a massive, highly aggressive empire with thousands of warp capable vessels, encompassing over 800 planets across a territory spanning some 16,000 light years (5,000 parsecs), or approximately twice the size and four times the controlled worlds of the United Federation of Planets as of the late 24th century. Obrist, a senior officer on the weapon-ship, reported this as a 98% restoration of the timeline. Their vessels were much more powerful, capable of inflicting severe damage to the Intrepid-class vessel.
Following an error in a temporal incursion, the Krenim were reduced to a pre-warp civilization, confined to their home system, with only a handful of small vessels.
The weapon-ship was finally destroyed by Captain Janeway and Voyager, and the original timeline was restored - the Krenim being a moderate power in dispute with the races around them. (VOY: "Year of Hell", "Year of Hell, Part II")