These were beasts of vessels, large and well defended. The experiences of the General War and the ongoing Cardassian War had taught Star Fleet some valuable lessens. Namely that although Star Fleet's primary mission was space exploration its role as the first line of defense of the Federation member worlds could not be trivialised. Too many of Star Fleets best and brightest had been lost at the hands of the Federation's aggressive neighbors. It seemed that Star Fleet had finally learnt this lesson the hard way.
It's new "explorer" category vessels would be operating for extended periods away from resupply. They would be exposed to unknown dangers and they would need to be able to protect themselves. These new vessels were in the size range of the dreaded Klingon B10 battleship. Advancement in warp propulsion, computing speeds and defensive capability permitted Star Fleet to produce larger vessels than it had ever done before. Such vessels could accommodate many more people on board than was required to operate the vessel. Being designed to spend extended periods away from any Federation base or member world, Star Fleet envisioned these vessels to act in a sense as mini villages, complete with families of the crew on board. The Ambassador class was the first Star Fleet vessel to experiment with this notion, but the Nebula and Galaxy class of vessels would really take this idea to the next level.