Long before we knew the title of Avengers: Endgame, the film’s directors Anthony Russo and Joe Russo pointed out that the title of the climactic Marvel movie was a reference to a line in Avengers: Age of Ultron.
Now, we know that Avengers: Endgame‘s title is a reference to a line Tony Stark says in Age of Ultron during an argument with his fellow superheroes when he points to the sky and says, “That up there, that’s the endgame.”
In Avengers: Infinity War, after relinquishing the Time Stone, Doctor Strange repeats this line to Tony Stark, telling him before The Dusting that “we’re in the endgame now.”
One Reddit user noticed this week that this might be more than a callback to the previous movie, and, in fact a clue for Tony to figure out how to stop Thanos.
Strange saw that Tony was going to have to go back in time, but he had to give him a hint as to when to go to. Her couldn’t outright just tell him without breaking a time continuum (or just a time travel movie rule).
Ultron will be a key to stopping Thanos. Ultron had just come out and escaped into the world. He was meant to be the key to stopping the Endgame in Tony’s mind. Tony might go back in time and fix Ultron when the party is going. When Ultron was created Tony even said to Bruce in disbelief “Were we even close to an interface?” He could have already gone back in time and was a catalyst to creating Ultron. Or someone else did and Tony will stop them.
Fans have theorized for some time that in Endgame the Avengers must somehow use time travel to go back and stop Thanos. Acorrding to leaked set images, and clips from the trailers, it looks like the Avengers will go back as far as Age of Ultron. Now, that could be because they can harness some sort of tool—like Ultron itself—to stop Thanos.
As we know, Strange spent time looking at thousands of possible outcomes of the war with Thanos and found only one scenario in which they win. That is why he decided to give the Time Stone to Thanos—because he needs to do exactly what he saw in this specific scenario. And that’s also why he gave Tony a clue to when in time he’ll need to travel to.