What was the motivation behind the conspiracy in Star Trek VI?

Question

Toward the end of The Undiscovered Country, it is revealed that

the Humans Admiral Cartwright and Colonel West, Klingon General Chang, Romulan Ambassador Nanclus, and Vulcan Lieutenant Valeris, possibly among others

were cooperating to prevent the signing of the Khitomer accords. The latter claims she's working to protect Starfleet, but it doesn't completely make sense why she'd be closely cooperating with enemies of Starfleet to do so. All of those participants have the same goal, but the reasons behind that goal is some pretty basic distrust of each other.

So how did this conspiracy happen? Was she lying about their motives?

Answer 1

I'm not aware of any reason to suspect that Valeris is lying. Her explanation is consistent with remarks made by several characters in the film. The motives of the conspirators are explained in the same remarks.

• The Romulans want to keep two competing powers hostile toward each other.

AMBASSADOR NANCLUS "Mr. President, they ARE vulnerable. There'll never be a better time..."

• The Klingons are concerned that their society will be subsumed by the Federation.

UNNAMED KLINGON GENERAL: "Attack or be slaves in their world!"

and

KERLA: "Better to die on our feet than live on our knees..."

• The Starfleet officers are worried that the federation will invest fewer resources in defence if hostilities toward the Klingons are ceased.

SPOCK: "If the Klingons sue for peace, we could accumulate savings in defense expenditures, leaving the Federation economy free to grapple with urgent social problems..."

A MILITARY MAN: "Bill, are we talking about mothballing the Starfleet?"

CinC: "I'm sure our exploration and science programs would not be affected, but the facts speak for themselves, Captain."

Quotes from here.