Wrong. The Requiem was first mauled over in the first months of the war (first battle of Neptune).
But not destroyed - and only lost a Deimos escort, while the Federation lost and rebuilt 7 frigates in the same timespan.
I was under the impression that it was the Meridian that assaulted Neptune, not the Requiem? And that the 3rd Fleet ambushed the Requiem in a separate engagement?
The First Battle of Neptune resulted in the loss of two frigates and four Sanctus cruisers for the UEF, one Deimos and lots of strike craft for the Tevs. A massive close-assault by gunships forced the Meridian to retreat, which is where they lost a Deimos escort.
Third fleet is mauled, second fleet is damaged...
3rd Fleet is still capable of engaging in major operations alone, like the taking/destruction of the Carthage.
2nd Fleet, according to Noemi, is just 'bloodied'
Bloodied was now Noemi described it early in Tenebra, IIRC, which was before Second Fleet lost at least one frigate and more strike craft, along with whatever Phobos Station was.
3rd Fleet isn't even a battlegroup at this point, though it can still do a lot of damage if used in the right place and the right time. At this point they're down to the Toutatis and its strike wings, two Narayanas, possibly a Karuna, one Sanctus, and maybe one Oculus (it might have been 2nd Fleet's Oculus). Quite formidable, yes, but by itself no match for a Tev battlegroup. At this point, if the Toutatis can be tied down by a next-gen destroyer or hunter-killer team, then a Hecate and a couple/few Deimos corvettes could easily overwhelm the remainder.
2nd Fleet is down to, what, the Eris, a few Narayanas, and a few Karunas (on top of however many cruisers and few Oculi it has left)? While quite formidable, it's also going to be facing a numerically superior force with beam cannons and tons of strike craft. It's also lost almost all of its best pilots, and Uriel gunships are scarce. Certainly not a forgone conclusion, but not exactly good odds either. The Tevs also have a lot more AWACS ships than the UEF does, especially when you factor in Aruroras.
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In general, I think the Tevs are better off with a strategy of gradually taking down the UEF fleet frigate by frigate, rather than waging a single massive battle. A war of attrition favors them, and as the numerical and technological superiority continues to shift in their favor, the pace of whittling down the UEF fleets increases. By taking advantage of Byrne's conservatism, they can commit an overwhelming force to destroy a frigate or two in 2nd and 3rd Fleets. Using a highly mobile force like Serkr, Steele could avoid escalation by making retaliation still very risky.