In "Time's Orphan", Worf makes this reference to the Kelvans, first seen in the Original Series. However, the Kelvans were from the Andromeda Galaxy, which was dying from radiation. They sent out probe ships around the 1960s to nearby galaxies. And therein lies the problem. The Kelvans that Kirk met were from one multigenerational ship, who took 300 years to get to the Milky Way, and tried to hijack the Enterprise for another 300 year journey to report back for the eventual invasion. Therein lies the problem. It takes three centuries for Kelvans to get to the galaxy, DS9 is only one century later, and Kirk convinced/punched their leader into accepting peace. So how does Worf fight Kelvans?