'Yesterday's enterprise' made clear enough what happened between Khitomer and Narendra III:
Khitomer turned a cold war into a frozen war.
The situation deteriorated without enterprise c's sacrifice for the klingons.
Open war broke out.
The klingons bitch-slapped the federation.
Details beyond these are nowhere established.
A probable scenario:
A large part of the fleet was mothballed after TUC (as per canon TUC, this was implied to be a serious possibility - and not contradicted).
The klingons recover from Praxis.
Starfleet training is reorganised to limit military training, to instill in starfleet personnel a distaste toward honing one's military skills.
This is made obvious in Picard, Riker's (leaders of THE top starfleet crew) behaviour from 'Peak performance'. See also Picard, Riker's airhead combat performance in episodes like 'Rascals' and others, proving they cannot use effectively the combat capabilities of their ship.
No Narendra III. The situation between the klingons and the federation deteriorates.
As per 'Yesterday's enterprise', the klingons killed 40 billion. You can't do that by killing merely military personnel.
Most likely, the klingons massed their forces for a surprise attack. Starfleet intelligence, as redundantly shown in canon, was/is utterly incompetent and failed to discover this.
The klingons launched their attack, destroying many federation worlds with populations numbering in the billions, eliminating large parts of the federation infrastructure.
By the time starfleet manages to mass its dispersed fleet to respond/starfleet officers manage to come up with behaviour distinguishable from utter incompetence, the klingons have a substantial advantage.
The federation is incapable of recovering during the war. Perhaps because many worlds seceded from the federation, not wanting to be the next world the klingons burn to a crisp. This is even more probable when one considers how different the cultures of the federation member worlds are.