One of the more talked about aspects of Avengers: Endgame's ending was the final fate of Steve Rogers. Following Thanos' defeat, Captain America used Pym Particles to travel through history to return the Infinity Stones and Mjolnir to their original points in timeline, ensuring its integrity. However, rather than using the Pym Particles to return to the Marvel Cinematic Universe's relative present, Steve traveled back to the 1940s to live out a life married to Peggy Carter, reappearing as an old man to pass the mantle of Captain America to Sam Wilson.
Since the revelation of the older Steve Rogers and his surprise use of time travel, there has been an enduring debate about what exactly Steve's actions entailed for the MCU's prime timeline. After all, Professor Hulk explained on two separate occasions in Endgame that every action by time travelers in the past would create its own divergent timeline as a sort of butterfly effect. The lingering debate about what actually occurred and the full extent of Steve's impact on the prime timeline would lead to directors Joe and Anthony Russo disagreeing with screenwriters Christopher Markus and Stephen McFeely, who felt Captain America's actions had always been part of the prime timeline, thus not creating a divergence.
In an interview to promote the special theatrical re-release of Endgame, Joe Russo went into detail about how he and his brother interpreted the fate of Steve Rogers and his effect on the MCU prime timeline. According to the directors, Steve traveled back to a point following his past self going into the ice to save New York City from the Red Skull, as seen in Captain America: The First Avenger. Despite being missing and presumed dead, Rogers quietly reunited with Peggy Carter, leading to their eventual marriage and life together, with the original Steve Rogers remaining preserved in ice until his discovery in 2011.
Joe Russo stressed this alteration of history would create a divergent timeline, where Steve Rogers lived out his life with Peggy, while the writers had thought this happy ending was always the case. From there, the older Steve Rogers would have to use Pym Particles or some other means to travel back to the prime timeline to reunite with Sam and Bucky Barnes to pass on the vibranium shield from this new timeline, as the original had been destroyed by Thanos during the final battle.
Massive divergences in the prime MCU timeline are not isolated to Captain America's actions during the final scene of Endgame. The 2011 incarnation of Loki used the Tesseract to escape to parts unknown following the Battle of New York due to Tony Stark and Scott Lang's tampering with the timeline, a sharp departure from the final fate of the God of Mischief and Space Stone in the prime timeline. The 2014 versions of Thanos and Nebula were both killed in the final battle after traveling to the MCU's present while the 2014 incarnation of Gamora is still on the loose years into her relative future creating further significant divergences. Presumably laying low for decades since returning to the 1940s, Steve Rogers' tampering with the timeline is actually a much less wild divergence from the MCU's original history.
What exactly Steve Rogers did in this divergent timeline and how he obtained Pym Particles to travel back to the prime MCU timeline decades later are, by Joe Russo's own admission, another story for another day. However, the co-director's explanation does make a lot of sense, especially as the MCU appears poised to explore alternate realities in the fallout of Avengers: Endgame. With Captain America happily married to Peggy Carter in another timeline, it will be interesting to see if the door is open for the Star-Spangled Avenger's potential return in the future.
Directed by Joe and Anthony Russo, Avengers: Endgame stars Robert Downey Jr. as Iron Man, Chris Evans as Captain America, Mark Ruffalo as Bruce Banner, Chris Hemsworth as Thor, Scarlett Johansson as Black Widow, Jeremy Renner as Hawkeye, Brie Larson as Captain Marvel, Paul Rudd as Ant-Man, Don Cheadle as War Machine, Karen Gillan as Nebula, Danai Gurira as Okoye and Bradley Cooper as Rocket, with Gwyneth Paltrow Pepper Potts, Jon Favreau as Happy Hogan, Benedict Wong as Wong, Tessa Thompson as Valkyrie and Josh Brolin as Thanos. The re-released version of the film containing bonus post-credits content is now in theaters.