(covers information from several alternate timelines)
The USS Voyager encountered a patrol ship during negotiations with the Zahl. The patrol ship believed Voyager to be in Krenim, rather than Zahl, space and opened fire. The patrol ship did no damage to Voyager and retreated.
When the timeline was changed by the Krenim weapon ship by erasing the Zahl, a temporal shockwave erased the Zahl starships and transformed the Krenim patrol ship into a Krenim warship. (VOY: "Year of Hell")