Although Spock is known for his good judgement, this trait is conspicuously absent in The Undiscovered Country. From his misguided trust of Lieutenant Valeris to his hastiness in furthering the peace process, poor Spock just can't get things right this time around.
Spocking Ain't Easy
That's not to say that it's all bad, however. Besides Gorkon, Spock is the main driving force behind the peace negotiations, having secretly met with the Klingons to hash out peace terms. He deserves all the props in the world for this.
The problem comes when he decides to make Kirk the Federation's representative without asking Kirk first—even though he knows that his friend hates Klingons with a fiery passion. But this is all part of the plan, says Spock. Take a look:
KIRK: We volunteered?
SPOCK: There's an old Vulcan proverb. "Only Nixon could go to China."
KIRK: How could you vouch for me? That's an arrogant presumption.
SPOCK: My father requested that I open negot—
KIRK: I know your father's the Vulcan Ambassador, for heaven's sake, but you know how I feel about this. They're animals.
SPOCK: Jim, there is an historic opportunity here.
Paradoxically, Spock thinks that Kirk will be a good negotiator because Kirk hates Klingons. That's why he compares him to Nixon. Nixon was the first American president to visit and establish relations with the People's Republic of China. Many people think he succeeded in his negotiations largely because he was seen as "tough" on the country. While there might be some wisdom behind this strategy (and who can argue with the results?), it leads to quite the complicated situation.
In contrast, Spock's mentorship of Lieutenant Valeris is a caveat-free fail. These two go way back: Spock was her sponsor in Starfleet Academy. Even more significant, however, is his decision to make Valeris his successor on Enterprise, as this will be his final mission aboard the ship. He must respect her a lot to entrust her with that responsibility.
So you can imagine how shocked he is when Valeris is revealed to be part of the conspiracy to thwart his peace negotiations. That would be like Obi Wan finding out the Luke was a Sith the whole time. Spock is thrown for a total loop:
SPOCK: I was prejudiced by her accomplishments as a Vulcan.
KIRK: Gorkon had to die before I understood how prejudiced I was.
SPOCK: Is it possible that we two, you and I, have grown so old and so inflexible that we have outlived our usefulness? Would that constitute a joke?
Although he might not be dealing with the issues as directly as Kirk is, Spock is going through a similar struggle now that his time with Enterprise is winding down. Being betrayed by the woman he chose to carry his legacy is like pouring salt into the wound.
The movie ends without definitely charting the course that Spock's life will take in his retirement years, but we're confident that he'll use them well. In fact, we wouldn't be surprised to see him chilling on some interstellar beach, mind-melding with all the eligible alien bachelorettes. Not that we've written a fan-fic to that effect or anything.