Natasha Romanoff's (Scarlett Johansson) decision at the end of Captain America: Civil War ending. Joe and Antony Russo's 2016 movie set the stage for the culmination of the Infinity Saga by breaking up the Avengers. Marvel Studios revisiting that tumultuous time period via Black Widow has offered new context about the film, particularly regarding the way things ended between Steve Rogers (Chris Evans) and Tony Stark (Robert Downey Jr.).
The main premise of Captain America: Civil War revolves around Bucky Barnes (Sebastian Stan) killing his parents.
The heroes couldn't find a middle ground, ultimately resulting in the temporary break-up of the Avengers. For two years the team spent their time separately, until the arrival of Thanos (Josh Brolin) forced multiple factions of heroes back into action in Yelena Belova (Florence Pugh) in liberating the other Black Widows. It's not outside the realm of possibility that it was Black Widow herself who convinced Steve to send his old rival the message.
Despite this, Tony and Steve remained separated in the two years that followed, and even during Thanos' initial attack in Avengers: Infinity War the two heroes were fighting on different fronts. It wasn't until Captain Marvel (Carol Danvers) saved Iron Man from dying in space in Avengers: End Game that the two were reunited back on Earth. Even so, it makes sense that Black Widow was the one to convince Steve to reach out first. It's unclear if Nat knew about Bucky's involvement in the death of the Starks—although given her association with SHIELD and closeness to Steve it's possible she found out at some point. Either way, she'd spent enough time with Tony to know that he would never initiate .
In addition to that, she could've also felt a little bit guilty for abandoning Tony on the heels of Rhodey's (Don Cheadle) accident. So, she tried resolving her own feelings and the whole mess, in general, by urging Captain America to take the next step to reconciliation. While Nat was typically stoic and indifferent, it's safe to say that the break-up of the Avengers in Captain America: Civil War impacted her deeply. Her experience in the Black Widow standalone film only further motivated her to reunite the Avengers—even if ultimately she wasn't directly responsible for putting Captain America and Iron Man back together again.
