You mean the Omega Class Destroyers? Yup. The Agamemnon was stated as one of the first to be built in the 36 hours on Babylon 5 episode and the thing a few think was an Omega class in Into The Beginning was, in fact, Nova class. If we look at all human designed big ships after the Mimbari War, whether the Omega class, the Warlock Class (now with added shadow technology), or even the Excalibur and Victory they have one obvious design goal. To have a big enough main gun (and preferably good enough targetting) that there isn't a ship that's too tough for them to handle. Although superficially a similar shape, the Hyperion and the Nova have much more balanced armament being designed to handle all comers rather than wreck Mimbari (and possibly First One) ships head on.I'm 90% sure the Agamemnons are a post-Minbari War development. Certainly I can't remember a single one in In the Beginning, and if there was ever a time to use the biggest, baddest ship in your arsenal, it's when you're fighting for your continued existence as a species. I also have a vague recollection of seeing it mentioned somewhere explicitly, but I couldn't begin to tell you where.
Edit: The lack of rotating front end allows the Nova class to get much more use out of guns mounted all along the side, and I can see them able to wreck e.g. Sun Hawks much more easily than the Omega class can. On the other hand we know how the Nova class did against Mimbari Sharlin-class cruisers.
Not even that advanced. One of the big carrots Delenn offers is artificial gravity - something the Mimbari and the Centauri already have. Most League worlds are little more advanced than Earth and behind the Centauri, never mind the Mimbari.Then the Dilgar happen. A power roughly on a level with the Centauri, technologically, invading the territories of the Legaue worlds. Most of the league worlds are advanced but inward-looking.
This. And so what if most of the war is fought with 1950s technology? If someone comes in with Tankograd, an army filled with T34/85s and the ability to produce them fast who cares that they are slightly obsolete tanks? Quantity has a quality all of its own - and good design intended for combat rather than low maintainance erases some of the quality gap.What's really needed to win this war for the League side, though, isn't high tech, it's manpower. They need lots and lots of ships.
Which is where Earth comes in. Not like the USA in WWII, but like the USSR.