The Federation in DIS was entirely inept to go even
near a war - despite being the most militarized portrayal of Starfleet
ever, with probably the exception of DS9 right during the middle of an actual galaxy-spanning war.
- They managed to almost let their one(!) dilithium crystal mine fall in klingon hands. If there exists one colony that could lead to defeat, that's the one you protect the most. Even DS9 got that right, where they put the wormhole over friggin' Earth(!) because of it's tactical value
- They get utterly pummeled by the cloaking device, but they have a way to defeat it - the crystalline thing-y on Pahvo, and they send one(!) ship, only once(!) - Then the next nine month nothing. Despite being a fuckin' science organization, and the klingons a scattered mess, not once does anyone in the entire Federation manage to crack them.
- They lost to a non-united army! That's...stupid as hell. If the klingon Empire is THAT much more powerfull, a united klingon Empire (like during the TOS era) should have been able to wipe out the Federation effortless - cloaking device or not (which, just btw, they did have then as well)
Basically, I think having the Federation so easily loose against the klingon was a
massive mis-calculation on the writers part. The klingons aren't the dominion or the Borg. They should be peers, even a bit weaker than the Federation, but still a formidable enemy.
The TOS Federation wouldn't have been that cocky towards the klingons - Kirk and the Enterprise being sent alone(!) to confront a klingon fleet(!), or Starfleet Admirals conspiring
for war with the klingons in 'undiscovered country', if the Federation basically has
NOTHING to oppose an aggressive klingon Empire and the experience of a devastating defeat already just a few years back.
Basically, the entire interpretation of the war against the klingons in DIS is
borderline unbelievable, and as incongruent with existing canon as the depiction of klingons in all else aspects (visuals, charcterisations, stories,...). Basically, it is best to pretend it never happened, like the Warp-10 salamanders in "Threshold". And in the last 10 minutes of DIS, they already went out of their way to basically erease everything that happened in that war from their memories and basically hit the reset-button full power, despite not technically resetting things actually, just pretend they would have no consequences.