Spoilers ahead for Avengers: Endgame.
Chris Evans' Captain America seemed to get a finale for his overarching Marvel Cinematic Universe storyline in Avengers: Endgame, though it left some questions about the ramifications of his decisions. And now, co-director Joe Russo has some definitive answers for how things went down.
The final scenes of the Endgame establish that, after using time travel to return the Infinity Stones to the past, Steve Rogers retired as Captain America instead of returning to the core timeline, and lived a full life into old age alongside his World War 2 flame Peggy Carter - appearing as an old man to pass on his shield and mantle to Sam Wilson in the present day.
But how did he wind up there - both dancing with Peggy in the non-core timeline, and back in the core timeline's present - and what does this mean for the past of the Marvel Cinematic Universe?
“If Cap were to go back into the past and live there, he would create a branched reality,” Joe Russo explained to Entertainment Weekly. “The question then becomes, how is he back in this reality to give the shield away?”
“Interesting question, right?” Russo continued. “Maybe there’s a story there. There’s a lot of layers built into this movie and we spent three years thinking through it, so it’s fun to talk about it and hopefully fill in holes for people so they understand what we’re thinking.”
The question of alternate realities and branching timelines is at the heart of one of Endgame's biggest conundrums - how to visit the past without changing the future.
Several scenes in Endgame establish the "rules" of time travel for the MCU. Present day characters visiting the past can't change what's already happened - their future can't change their own pasts. However, changes in the past can create branching, alternate timelines.
Doctor Strange envisions millions of alternate futures in Avengers: Infinity War, and in Endgame his mentor. the Ancient One, establishes in conversation with Hulk that failing to return the Infinity Stones to the moment they were taken from the past will result in branching timelines where billions die - which would ultimately have the same effect as Thanos's snap, in terms of lives ended.
So Steve returning to the past and being with Peggy establishes an alternate history of the MCU, leaving the past we know (where Steve was frozen and Peggy moved on) intact - but creating a parallel world where he returned and they had a life together.
Of course, as Russo pointed out, that leaves the question - how did Steve Rogers return to the core reality to pass on his shield to Sam? And could the answer to that question relate to Joe Russo's previous comments that Chris Evans wasn't technically done as Captain America after Endgame?