For five long years, the Avengers struggled to find a way to undo Thanos’ act of wiping out half of life in the universe. Despite the combined resources of heroes and civilizations across the universe, it seems impossible to do anything but deal with the loss and try to move on.
Then Scott Lang (aka Ant-Man) joined the Avengers, babbling something about the “quantum realm” and time travel. Tony Stark did some math and discovered there was a lot of truth to Scott’s confused words, and suddenly the Avengers had a chance to go back in time, get the Infinity Stones before Thanos and undo snap effect.
It was an extremely risky and difficult plan. Not only is there little chance for it to actually work, but the Avengers run the risk of losing even more members to unproven time travel technology. In the end, as the new leader, Natasha decided to go ahead with the time travel plan as a last-ditch effort to save the world. Fortunately, the gamble was successful, although it took exactly the biggest personal cost from Natasha.
https://www.looper.com/729614/the-best-decisions-black-widow-made-in-the-mcu/ Black Widow’s Best Decisions in the MCU