Discovery has a war with the Klingons that TOS did not have.
Says who?
Nobody says there had been no previous war with the Klingons in "Errand Of Mercy" (TOS). At its outset,
"negotiations with the Klingon Empire are on the verge of breaking down" and, when they then do, the comment is:
"War. We didn't want it, but we've got it." Nothing about that directly implies a lack of prior conflicts a decade earlier. If anything, the opposite, I should think.
As was pointed out by our dearly departed
@GSchnitzer, the
first draft of the script for "Day Of The Dove" (TOS), dated 9 August 1968,
did contain this line:
MCCOY: [sour] Fifty years -- eyeball to eyeball with the Klingon Empire. They've spied -- raided our outposts -- pirated merchant lanes. A thousand provocations, and the Federation has always managed to avoid war. Now, this crazy business could pull the trigger!
...but it was subsequently
cut before ever being filmed!
In "The Infinite Vulcan" (TAS), Kirk says
"there has been peace in the Federation for over 100 years" but this claim is immediately slapped down by Keniclius, who retorts:
"That is a lie! What about the Eugenics Wars? The galactic wars? What of the depredations of the Romulans, the Klingons, and the Kzinti?"
In TWOK, Carol Marcus says
"Starfleet has kept the peace for a hunderd years" but this is clearly in context of the conflict David raises between "scientists" and "the military" and nothing else.
In "First Contact" (TNG), Picard says: "
Centuries ago, disastrous contact with the Klingon Empire led to decades of war..."
In TUC, Spock says there has been
"almost seventy years of unremitting hostility" between the Klingons and the Federation.
So the Klingon War was ultimately precluded by
nothing in onscreen canon.
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MMoM