Dwellers in the Crucible (Star Trek: The Original Series #25)

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!