Apologies for the long post
Tyger, I think you've got it right on most points.
I have to bring you up on this one though!
Originally posted by Sea Tyger
Destroyer escorts have been around almost as long as destroyers, eventually being called "frigates" at some point after WWII.
That really only applies to the USN and US-influenced navies. The Royal Navy has been using frigates since the 18th century at least. However, the term seems to mean something different in the USN parlance - in the RN, a frigate is the smallest ship of the line, with the destroyer being the next one up, then cruiser, then battleship. In the US, a frigate is usually bigger than a destroyer, and I seem to remember reading somewhere, it has a very specific role and size.
Getting back on topic
The evolution of types is something I think would continue through the life of Starfleet.
During the Romulan War, they probably only had a few basic designations, as ships operating a long time out of port would be heavily multi-role, so you'd probably see a cruiser, a destroyer and maybe a battleship. Fine distinctions like "heavy" and "light" or specialisations like "escort" would be irrelevant as they would place limitations on the use of precious battlefield resources. All vessels would be as heavily armed as they could manage.
By Kirk's time, we have a frontier situation where again, vessels are a long time out of port, and need to be multi-role. I always saw the Constitution as the flagship of the fleet. While it has a scientific role, it is also a military vessel (often the only one in the area), and a diplomatic vessel, so it had to be both powerful and capable of "showing the flag" (rather like the old imperial gunboats of the 19th century). The designation of "cruiser" is appropriate for such a multi-purpose vessel - it engages in long-distance cruises - and the type name is not threatening. Other specialised vessels would exist, including sub-cruiser military types (I'd imagine the frigate exists at this time - primarily as a less aggressive-sounding designation for a destroyer), but none would be as high-visibilty as the cruiser.
In the movies, the political situation is changing, the continuing aggression of the Klingons has forced a proliferation of military types (and IMHO a more military Starfleet). The Miranda and Excelsior classes have appeared (the former heavily armed) and there appear to be plenty of others. More type designations appear as vessels and roles appear to fill them. I suspect we now see escorts (again used as opposed to destroyer) and scouts making a serious impact in the shipyards.
Over the next hundred years, things calm down a bit. Starfleet returns to it's primary role of exploration, and the first true explorers appear. I might hazard a guess and say that the Galaxy is the first class of this type, and that maybe the type was named in honour of the original Enterprise NX-01 - which I would say is primarily an exploration vessel. Starfleet continues to fight wars (Romulans, Klingons, Cardassians) but none are threatening, so cruisers, frigates and escorts continue to exist in specialised variants, but not in great numbers. Explorers, scouts and surveyors appear in great numbers.
The Dominion War swings it all back the other way. Military vessels suddenly become important, so we see a proliferation of cruiser, frigate and escort classes. Non-military types gain higher weapon ratings. The only reason we don't see a battleship is development time. I dare say the Sovereign was designed as an explorer - a companion to the Galaxy, but then was retrofitted with higher firepower and a role change (I'd definitely put it in the heavy cruiser category now).
Note that I don't agree with the Defiant being the first warship designed by Starfleet, it just doesn't make sense. However I can see that it might be the first pure warship designed in the preceding fifty or sixty years.
Just a few thoughts!
Jon
"There are worlds out there where the sky is burning, where the sea is asleep and the rivers dream; people made of smoke and cities made of song.
Somewhere there's danger, somewhere there's injustice, and somewhere else the tea is getting cold. Come on, Ace, we've got work to do."
THE DOCTOR, "Survival" (Doctor Who)