Months have passed since Avengers: Endgame was finally released, but a new clue suggests that the first Iron Man movie explains why Doctor Strange knew Tony Stark had to be the done to ultimately defeat Thanos. Released in 2008, Iron Man was the beginning of the MCU as it’s now known, and introduced Robert Downey Jr. as Tony Stark, as well as the origin of his armor and eventual superhero identity, Iron Man.
Iron Man went on to appear in nine more films in the MCU (without counting his cameo at the end of The Incredible Hulk), with Avengers: Endgame marking the end of his arc. Tony Stark sacrificed himself in order to save the world, and was the one who made the final snap, which wiped out Thanos and his army. In Avengers: Infinity War, Doctor Strange saw all possible outcomes and revealed to Tony that there was only one in which they won. Strange knew Tony had to be the one in charge of the final snap – but has the reason for it been out there all this time?
A Reddit user noticed a possible connection between a scene in Iron Man and Tony’s sacrifice in Avengers: Endgame. The author shared screenshots from Iron Man when Tony and Yinsen are building the arc reactor and the very first Iron Man suit – aka “Mark I”. Yinsen points out that the arc reactor could run his heart “for 50 lifetimes”, to which Tony replies that “or something big for 15 minutes”. This has led fans to believe Tony was able to hold the Infinity Stones thanks to the arc reactor, as it took the stones’ energy, allowing him to snap his fingers. The Hulk was the one who reversed Thanos’ snap as he was the only one who could take the stone’s power without dying, although it left him with permanent damage. Because of this, many fans have wondered how Tony was able to hold them, say his famous “I am Iron Man” line, and snap his fingers without being incapacitated.
Tony’s line about the arc reactor being able to run something big for 15 minutes isn’t exactly a clue or Iron Man foreshadowing Endgame, as he was referring to the “Mark I” suit, and the reactor is more a source of energy that one that receives it. Tony was able to do what he did because the nanotech in the gauntlet they made and the one in his suit were the same, and so the energy of the stones was handed off to the suit, which protected him and kept him alive while holding them. Tony Stark was the one who had to do the final snap not only for narrative reasons (the hero that started it all was the one who made the ultimate sacrifice. And Robert Downey Jr’s contract was over), but because he was the only one literally built for it – and Doctor Strange knew it.
While it’s not impossible that the arc reactor played a part in it (by working with the suit to keep him alive and protect his heart from the stones’ power), it seems unlikely that it was the one reason why Tony could wield the stones. Doctor Strange, in the many possible scenarios he saw, surely witnessed other heroes trying and failing to wield the gauntlet and the stones, and knew that Tony Stark/Iron Man was the only one with the resources to do the final snap, even if the price was too high.