Way back around the "Battle" of Wolf 359, Commander Sisko helped design a starship specifically intended to fight the Borg. No holodeck or other luxuries, just a ton of weapons. Because the Borg threat decreased, the project lost relevance for a few years.
A few years later, while helping his son with his homework, Sisko first encountered agents from the Dominion, a threat that's just as if not, more deadly than the Borg. After the Dominion showed how dangerous they were by kamikazing a ship that's the same class as the Enterprise-D, the anti-Borg ship was dusted off.
Not only did Starfleet still try to pass this ship off as a simple escort vessel with a ton of guns and torpedo launchers, they managed to make a deal with the Romulans and get a cloaking device through a loophole in a peace treaty. How does Sisko introduce this groundbreaking ship to his crew? By sneaking up to Deep Space Nine while cloaked and scaring the shit out of everyone.
Eventually, total war breaks out between the Federation and Dominion, and rather than the shiny new Enterprise-E, the tiny "escort" ship is the vessel that sees the most action until it's destroyed by the Breen... and is replaced by a nearly identical ship also called the Defiant.
