Dwellers in the Crucible — Margaret Wander Bonanno

Warrantors of Peace: the Federation's daring experiment to prevent war among its members. Each Warrantor; man or woman is hostage for the government of his native world--and is instantly killed if that world breaks the peace.

Now Romulans have kidnapped six Warrantors, to foment political chaos--and then civil war--within the Federation. Captain Kirk must send Sulu to infiltrate Romulan territory, find the hostages, and bring them back alive--before the Federation self-destructs!