(covers information from several alternate timelines)
The Halkan homeworld had rich deposits of dilithium.
In 2267, Captain Kirk and some others in the USS Enterprise crew were trying to negotiate rights to mine the dilithium. When they were denied the crystals, they tried to beam up to their ship, but ion storms at the planet caused a transporter malfunction, which displaced Kirk and the others into an alternate universe. When there, they found the Federation crew replaced with a brutal Imperial crew. Spock's counterpart ordered a barrage of Halkan cities. When Kirk heard this, he contacted the Halkan Council and gave them twelve hours to decide, much to Spock's surprise. But Starfleet Command ordered Spock to kill Kirk if he didn't destroy the Halkans by dawn. However, his assassination attempt failed, and afterwards, he helped Kirk and the landing party get back to their universe. (TOS: "Mirror, Mirror")