Nob Akimoto blssdwlf responded to many of your arguments. I add the following: The 43 billion deaths could be federation+klingon casualties. If the klingon casualties exceeded a few hundreds of million, then the federation engaged too in planetary genocide, following the klingon example. One cannot reach 43 billion deaths by killing only military - in a mere 20 years. The federation has consistently been implied as having more resources than the klingons - territory, infrastructure, GDP, etc. The klingons destroyed much - perhaps most - of this during their surprise attack at the beginning of the war, capitalising on starfleet intelligence's incompetence and the starfleet ships being widely dispersed on exploration missions. What is left is more than enough to advance starship technology and build the gigantic enterprise D and many other ships - for example, Utopia Planitia/the Sol system alone is sufficient for this. And much more of the federation than merely the Sol system most likely survived the klingon initial attack. Enough to soldier on for 20 years in a death spiral. Not enough to win. After TUC, building new ships and upgrading ships is compatible to a drastic overall reduction of starfleet numbers in general and military ship numbers in special (more ships are mothballed/lost than built). The fact that by the 24th century we're treated to dozens upon dozens of Excelsior and Miranda, again, is compatible with a major overall reduction of starfleet numbers after TUC. Keep in mind that, by the 24th century, it's established the great powers had thousands of ships in their fleet - as per DS9. The fact that in the 24th century the federation chose to retrieve from mothballs/use one century old starfleet ship models on such a scale, as opposed to designing/building new ones implies a blatant disinterest on the part of starfleet in ship design/construction. Castillo all but confirmed the fact that starfleet officers had a lacking tactical/military training in his time when he acknowledged that Yar is superior than anyone on enterprise C (by Yar's time, altered timeline starfleet most definitely DID emphasize military training in its recruits). Tactical abilities were not required of Garrett to show up at Narendra III, survive while the ship shields held, then have the ship destroyed, the pieces/black box of the ship being found in the subsequent forensic investigation. Secondary border wars could be an additional reason for starfleet failing so badly by 'Yesterday enerprise'. Of course, considering how little they strained the federation/starfleet resources in the Prime timeline (Picard&co didn't even felt there was a border war with the cardassians going on during the first seasons of TNG), they are, at most, secondary to the federation's fall in the altered timeline. This is corroborated by the characters speaking only of klingons as the opposing power in 'Yesterday's enterprise'.