Daniels did say that the Klingons join the Federation by the 26th century in Enterprise, though the Federation and Klingons have had 2 previous conflicts with this being the third Federation/Klingon war, they need each other in face of the iconian threat, the dominion threat brought peace between these 2 super powers in 2373.
Maybe it's time for the Federation to swallow it's pride and admit the existence of undine infiltrators and get a negotiated peace and re sign the khitomer accords to bring this stalemate of a war to a end, the Klingons give up their conquests in romulan space and the federation give some of eta eridani as a buffer zone.
The Feds will never do anything sensible when the Feds are infiltrated by Undine themselves. Even after they discover a friggin' ADMIRAL in Starfleet was an Undine, nothing else is done about it, except in sending you to Romulan space to kill Romulans.
"Wait, how did that make sense?"
Obviously, killing Romulans is very much a valid course of action in finding the Undine within Starfleet and the Federation's ranks.
The Klingons had taken great (and bloody) pains in rooting out infiltrators, which could not have been a an easy or pretty task.