Well, not the Romulans yet...
I'd think any attacker capable of cloaking should be able to grab a lot of territory quickly. I mean, at close ranges, the Klingon cloaks here are like the Romulan one in "Balance of Terror": the presence of the invisible ship is evident, her movements trackable, but targeting is impossible. But at strategic ranges, Klingons (or, in other eras, Romulans etc.) can move their forces to whichever planet they wish, concentrating their attacks, while the Feds have to spread out to defend everything at once. (Indeed, is this how they got to those dilithium mines deep behind UFP lines?)
Losing to the Klingons in 2256 seems assured, as this is the first time the Federation fights an invisible enemy in a war of conquest. Sure, they know of invisibility as a thing, and may have played wargames about a Suliban invasion or something, but then dismissed those because the Suliban aren't attacking.
Nothing else about the Klingon attack seems unmanageable. Indeed, the Feds seemed to be winning hands down, with minimal Starfleet casualties and no mention of civilian ones, up until Kol distributed those cloaks to his allies.
Timo Saloniemi