Alexander Marcus

"If you've got something to say, say it. Tomorrow's too late."

- Alexander Marcus, to James T. Kirk

"You better stop and think about what you're doing, Kirk. You better think about what you did on Kronos. You made an incursion onto an enemy planet, you killed a Klingon patrol. Even if you got away without a trace, war is coming! And who's gonna lead us, you?! If I'm not in charge, our entire way of life is decimated! So you want me off this ship... you'd better kill me."

- Marcus's last words threatening Kirk, before Khan does the killing deed to the admiral himself.

Admiral Alexander Marcus was the hidden secondary antagonist (once Khan proved more dangerous than him) of the 2013 film Star Trek Into Darkness and was the head of Starfleet and a leading member of the Starfleet black ops unit known as Section 31.

He was portrayed by Peter Weller, who also played John Frederick Paxton on Star Trek: Enterprise.

History
Early in his career, Marcus was the first officer on an early incarnation of the USS Enterprise (between the NX-class Enterprise of the 22nd century and the Constitution-class Enterprise launched in 2258) under Captain Robert April. During the course of his career, he persuaded a young Christopher Pike to join Starfleet, while also acquiring a deep distrust of the Klingons.

Following the destruction of Vulcan in 2258, Marcus began mass sweeps of the galaxy and eventually found Khan's ship, the SS Botany Bay. Taking advantage of the genetically engineered human, he had Khan awoken and used his enhanced intelligence to build weapons for the Federation, his plans being to be prepared for war with the Klingons, seeing them as the Federation's greatest threat. To force Khan into compliance, Marcus held Khan's crew hostage. Khan tried to smuggle his crew out disguised in torpedoes he himself had designed; however, Marcus found out and confiscated the torpedoes, forcing Khan to escape alone and later go rogue, seeking vengeance for the Admiral's actions.

In the aftermath of Khan's actions, upon learning Khan was on Kronos, the Klingon homeworld, Marcus became paranoid, incorrectly believing that Khan was in league with the Klingons. He therefore devised a way to kill two birds with one stone, and sent Captain James T. Kirk, along with the torpedoes that Khan's crew were hidden inside of, with orders to fire the torpedoes at Khan from a distance, claiming they were specially-designed stealth torpedoes and would remove the threat without the Klingons noticing.

Kirk realized something was wrong and, heeding the advice of Commander Spock and Lieutenant Commander Montgomery Scott, he instead captured Khan, who revealed what Marcus had really done, including that the Admiral sabotaged the USS Enterprise to prevent them from escaping Klingon space.

Marcus then arrived in the USS Vengeance, a Dreadnought-class warship designed and built by Khan, planning to eliminate Kirk in order to cover up any evidence that it was the Federation, not the Klingons, who attacked first, giving himself the moral high ground in the anticipated war. Kirk managed to play along long enough for the Enterprise to jump to warp and try to escape Marcus, but the Vengeance proved to be faster and shot the Enterprise out of warp near Earth, causing massive damage and numerous casualties.

Kirk tried to save his ship by offering himself in the place of his crew, but Marcus coldly told him that he had always intended to blow up the Enterprise and that the war with the Klingons would mean he could turn the Federation into a military dictatorship like he wanted. It was then revealed that Carol Marcus, the Admiral's daughter, had sneaked aboard the Enterprise and she claimed he would have to blow her up with it, but Marcus simply transported her off the Enterprise and onto the Vengeance. Marcus then proceeded to fire on the weakened Enterprise, but before he could hit the starship with a crippling blow, the power went off on the Vengeance; Scott, as it turned out, had infiltrated the Vengeance and sabotaged the engines, giving Kirk a reprieve.

Marcus tried to respond to Kirk and Khan jumping across space and aboard the Vengeance, but was confronted by Kirk, who placed him under arrest for his actions. Holding him at phaser point, Kirk told Marcus to get out of the captain's chair, but the admiral refused, arguing that war would come now, and if he wasn't there to lead the Federation, it would be destroyed then tries to goad Kirk to killing him. Kirk didn't listen and told Marcus to either get out or he would stun him and drag him out.

But at that moment, Khan re-awakened (earlier, Kirk had Scott stun him once they got to the Vengeance's bridge), beat the two to the ground and broke Carol's leg. While the fight was ongoing, Marcus escaped to a console near the back of the bridge and attempted to activate the ship's auto-destruct sequence, which would have destroyed both the Vengeance and the Enterprise, erasing all evidence of Marcus's conspiracy and protecting his Section 31 colleagues. In his haste, however, Marcus entered his access code incorrectly, and before he could try again, Khan grabbed hold of the Admiral and killed him by cracking his skull.

Trivia

 * The life and history of Alexander Marcus may have been affected by the temporal transit of Nero and Spock Prime. In the original timeline, there was no indication that Carol Marcus had a highly-placed father in Starfleet. In fact, given the attitude of her son David towards the service, it even seems likely that she did not (or else the original Alexander Marcus also had a rather checkered career, though to probably less of an extent than his alternate reality counterpart).
 * Marcus is similar with Eli Damaskinos:
 * Both are seemingly appeared as an ally for the protagonist whom have them hunted down certain individual (Eli had Blade hunted down Jared Nomak while Marcus allowed Kirk tu hunt Khan.
 * Both Marcus and Eli are ironically killed by the individual that they sought, whom even proved to be more dangerous threat than they themselves. (Eli was killed by Jared, whom still wanted to purges vampirekinds while Khan sought to purge races that he deemed inferior than him after he killed Marcus)