The Final Decision was the Ekosian Nazi party's plan for genocide against all Zeon people, aliens from a neighboring planet, under the xenophobic propaganda that "the Zeon monsters" were on there "poisoning our planet."
Pursuit of this course began with sanctioning the "elimination" –the imprisonment, torture and execution, if not the outright beating to death on the street –of the Zeons who had colonized Ekos a half a century earlier.
After it was revealed that the capital had been made Zeon-free, the Final Solution was revealed with the plan to rid the entire solar system "of the disease that was Zeon," by destroying that planet by means a space fleet consisting of "both manned and unmanned weapons."
The Final Solution was halted mid-execution through the intervention of the landing party from the USS Enterprise, who where there to locate missing Federation cultural observer, turned Führer, John Gill. Although Zeon/Ekosian resistance pleaded with Captain James Kirk to order his ship to destroy the invasion fleet, Kirk opted for a better solution, by locating Gill, who had been drugged and manipulated by his Deputy Führer Melakon, and having him reverse the attack order on Zeon, before being assassinated by Melakon, who himself was also killed.