(covers information from several alternate timelines)
In 2267 on stardate 3739.8 Captain James T. Kirk and three of his crewmembers from the USS Enterprise had beamed down to the Halkan council in order to obtain permission to mine that planet's dilithim reserves. The Halkans refused, fearing their resources might one day be used in an unforeseen war. The Enterprise crew beamed up respecting their ethical views.
At the same time in the mirror universe Kirk's mirror counterpart from the ISS Enterprise had beamed down to that universe's Halkan council and threatened to destroy that planet with a phaser barrage if they didn't surrender their dilithium. The council refused, saying they would die as a race to preserve their history of total peace.
At the time there was a severe ion storm in both universes that overcharged each Enterprise's respective transporter, with the result that when the landing parties were beamed up they were swept into each other's universe. Kirk was forced to stall for time while under orders to annihilate the Halkans, and Spock was under orders to assassinate Kirk if he failed to comply.
It was because that Spock had the decency to warn Kirk of those orders that Kirk saw a chance to change the universe. Before beaming back to his own ship Kirk urged that Spock to take command of the Enterprise and try to steer the Empire toward a more civilized course. Spock acted on that advice and used the Halkans as a model for his reforms. Unfortunately this allowed the Halkans to win influence in Spock's government with their pacifist views, resulting in Spock's overthrow by the Imperial Starfleet in at least one universe and the Halkans' subsequent annihilation. (The Final Days)