A treaty doesn't automatically reverse decades of hostility and distrust. We have little information of what went on between TUC and YE, shifts in the internal politics of the Empire or Federation could have caused strains in their relationship.
The sacrifice of the E-C (a famous name to the Klingons as well) would show the Empire just how dedicated the UFP was to the alliance, that it wasn't just some kind of lip service as a means to keep the peace.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)