SPOILER WARNING: The following article contains major spoilers for Avengers: Infinity War. If you have not yet seen the film, please bookmark this page, and save the read until after your screening!
There are many pivotal, universe-changing decisions made by characters throughout Avengers: Infinity War, but unquestionably one of the most significant is the deal with the devil executed by Doctor Strange in the final 20 minutes. After the heroes fail to remove Thanos' Infinity Gauntlet, the battle on Titan begins to wrap up when the Mad Titan successfully stabs Iron Man through the chest with his own blade. It's a gut-wrenching moment -- but things somehow get even worse from there. Thanos taunts Tony Stark as he slowly dies, but stops when the Master of the Mystic Arts offers him a deal: the Time Stone for the life of the Avenger. It's an exchange that goes through without any tricks, giving the purple alien his fifth of six Infinity Stones, and paves his path towards accomplishing his ultimate goal.
Of course, on the surface this looks like an awful arrangement. While Iron Man was absolutely fundamental in the creation of the Marvel Cinematic Universe as a franchise, his life is worth nothing compared to half of all existence, and this is something that you'd think Doctor Strange would understand. The reality, however, is likely a lot more complex than that. While there isn't necessarily any firm proof right now, there is reason to speculate that the wizard is quietly playing the long-game towards victory.
The most important evidence of this is featured in the middle of Avengers: Infinity War, specifically when Iron Man, Star-Lord, Spider-Man, Mantis and Drax first head out to the surface of Titan. As the group gathers to form a plan, Mantis points their attention towards Doctor Strange, who is in a trance while using the Time Stone. When he awakens, he reveals that he has glimpsed 14,000, 605 possible futures, with only one resulting in success -- and it's this experience that one can use to explain Doctor Strange's terrible trade.
It's true that we aren't offered any details about the triumphant future that Strange glimpsed, but his knowledge of it could forgive his quick willingness to give Thanos the Time Stone. To put it in simple terms, whatever eventually happens, it's likely that Iron Man plays a crucial part, and he can't do that if he's dead. From a larger perspective, as crushing as the end of Avengers: Infinity War may be, it's entirely possible that Earth's Mightiest Heroes and the Guardians of the Galaxy needed to lose before they could win. Exactly how that victory is achieved is unclear, but all eyes will definitely be on shell head when the moment of truth eventually comes.
Truthfully, all of this is readable right in the dialogue. Shortly after his life is saved and Thanos makes his way to Earth, Tony turns to Doctor Strange and weakly says, "Why would you do that?" -- to which Strange replies, "We are in the end game now." And then there's Strange's last line of the movie: "Tony, there was no other way." In the moment he knew what needed to be done, and despite all appearances, set forward a course of action that should hopefully end with the heroes victorious at the end of Avengers 4. It's just too bad that he won't be around to guide everyone down the path, but that should only make things that much more exciting and dramatic.