In the alternate reality, Captain Christopher Pike gave a field promotion to Commander Spock, to acting captain, and Cadet James T. Kirk, to first officer. After Spock was revealed to be emotionally compromised by the destruction of Vulcan, he resigned his command, per Starfleet Regulation 619, which automatically field-promoted Kirk to acting captain. Kirk was formally recognized by Starfleet as the captain of the USS Enterprise, while Spock resumed his original rank and position as first officer. (Star Trek)