A preemptive strike is a military strike by one faction against another, believed to be planning an attack themselves.
In 2153, the Xindi launched a preemptive strike against Earth after being told, by beings from the future, that they themselves would be destroyed by the Humans. The probe they sent wiped out seven million people, carving a swath of destruction from Florida through Cuba to Venezuela. (ENT: "The Expanse")
In 2154, Ambassador Soval informed Commander Tucker that Vulcan Intelligence claimed the Andorians had developed a weapon based on Xindi technology and that they were planning on using it against Vulcan. Administrator V'Las used this claim to persuade the Vulcan High Command to launch a preemptive strike against Andoria, despite the peace treaty that was signed two years prior. (ENT: "Awakening")
Following Chancellor Gowron's demand that the Federation leave the Archanis sector, Jadzia Dax spoke to a friend in the Diplomatic Corps, who told her that several planets near the Klingon border were urging Starfleet to make a preemptive strike. (DS9: "Broken Link")