(covers information from several alternate timelines)
The Krenim were a technologically-advanced humanoid species from the Delta Quadrant, first encountered by the Federation starship USS Voyager in 2374. The Krenim government was called the Krenim Imperium.
In an alternate timeline, Voyager traversed Krenim space beginning on stardate 50973, enduring constant Krenim attacks in what the crew eventually termed the "Year of Hell". A large number of the crew was killed, including Captain Kathryn Janeway, B'Elanna Torres, and Joseph Carey. During one of the Krenim attacks, Kes was exposed to chroniton radiation which leaked through the hull from a chroniton torpedo fragment. Much later in 2379, The Doctor's attempt to extend her short Ocampan life span using a bio-temporal chamber activated the dormant chronitons inside her body, causing her to travel randomly backwards in time to various points in her own life. The Doctor of 2373 was able to purge the chronitons using anti-chronitons, synchronizing Kes to the year 2373. While she was in 2373, Kes warned Janeway of the Krenim danger. (VOY: "Before and After")
According to the Krenim officer known as Annorax, their greatest adversary was a race known as the Rilnar. Unbeknownst to the Krenim, however, the Rilnar had introduced a crucial antibody into the Krenim genome which prevented certain diseases from affecting the Krenim. In at least one alternate timeline, Annorax wiped out the Rilnar civilization without realizing the existence of the antibody and an epidemic swept across the Krenim homeworld, killing 50 million people in the first year alone.
In a number of separate alternate timelines in 2374, Voyager attempted to enter Krenim territory. Initially, they encountered only a small, poorly armed Krenim ship. In this timeline, the region was dominated by the Zahl, who had ended the Krenim domination of the region a generation previously. This timeline was changed when the Krenim weapon ship erased the Zahl civilization, removing the entire species from history. In this new timeline, the Krenim Imperium was near the height of its power, and launched a series of devastating attacks on Voyager much like the "year of hell" of Kes's possible future.
Eventually, Voyager developed temporal shields to defend against the Krenim, which in turn perturbed the temporal calculations made by the weapon ship. Thus, the elimination of the Garenor species, intended to further restore the Imperium, instead created a third alternate timeline where the Krenim regressed to a pre-warp society. The temporal shields protected Voyager from the changes to the timeline, and its crew was able to see the changes the Krenim had made. (VOY: "Year of Hell")
Eventually Voyager created an alliance with the Mawasi and the Nihydron to attack the weapon ship. The damage they dealt to the weapon ship caused it to erase itself from the timeline, undoing all of the damage it had done.
In this restored timeline, Voyager encountered a Krenim warship in 2374. Its commandant warned Captain Janeway that the region of space they were passing through was in dispute and that they should avoid it if possible. He wished them good journey and the two ships went their separate ways. (VOY: "Year of Hell, Part II")