Doctor Strange doesn't seem to get nearly enough credit in Avengers: Infinity War for engineering the one choice out of millions to spare the lives of half of the Marvel Cinematic Universe. Thanos already had four of the six Infinity Stones he needed to finish the Gauntlet, which he obtained by sacrificing his daughter. Thanos glanced out on his homeworld and saw Tony Stark waiting for him, determined to see it through while still grieving. Before speaking to his image of the cosmos, he confirmed his concerns that one of his other children, Black Maw, had died. The melee ensues shortly after, and their only true hope appears to be Peter Quill's plan. Surprisingly, they can pull it off flawlessly until Quill understands Gamora's fate. Starlord squanders their advantage by striking Thanos, dislodging Mantis and her sedating effect from his brow, and the Mad Titan unleashes his entire rage against the assembled warriors.
Thanos stabs Stark in the side before clenching the Gauntlet to inflict the last blow after hurling orbiting moons and shattering eldritch duplicates. Strange appears and informs Thanos that if he saves Tony's life, he will give him the Time Stone. Thanos informs the mage that any betrayals would be punished, and they reach an agreement.
Strange sends the Stone soaring toward Thanos, who immediately transforms it into an almost complete ruin and renewal artefact. Stark asks Strange why he chose that path as Thanos travels to acquire the sixth and final Stone, and the answer is intriguing. With the benefit of hindsight, Strange encased Thanos in an honour bomb and waited for it to detonate.
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When Strange travelled across the 14 million branching realities of Infinity War, the only one where Thanos was vanquished was the one in which Tony sacrificed himself to convert Thanos and his armies to ashes with the Iron Gauntlet. Strange wasn't pleading for Tony's life out of altruism; he was attempting to ensure that Tony would be alive to save everyone, including himself. The only flaw in his design, according to Thanos, is that the brilliance of his vision was intrinsically random. He didn't give a second thought to who lived and who perished; instead, he clicked his fingers and let chance decide the horrific details.
Strange examined two possible scenarios as a result of this. Tony had a 50/50 chance of surviving the Snap, or he knew Tony would survive after Thanos accepted his offer.Strange would have had significantly better odds than one in 14 million if he had chosen the 50/50 choice. Strange would have had to make an exception when he executed the Snap, singling out Tony specifically and thus breaking the beauty of his plan, if he knew Thanos would keep his word and spare Tony's life, which would mean he would have had to make an exception when he executed the Snap, singling out Tony specifically and thus breaking the beauty of his plan. The Gauntlet had previously proven that it can accommodate a wide range of nuance.
One of Tony's criteria for participating in the Time Heist was that after they recovered all six Stones from the various timelines, no changes to the previous five years would be made in order to protect his daughter's life. This was certainly achievable because Tony did just that when he made his Snap. Tony's was far more precise than Thanos', since he had to carve out a niche for himself to kill only the Titan's forces and no one else. It was also established that one person might be singled out in the casting, as the Hulk described when attempting to resurrect Natasha.
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He failed, although it does not apparent that it was because the complexities of one life were too difficult to parse with the Stones' power, but rather because obtaining the Soul Stone necessitated a sacrifice that could not be undone. Clint forewarned Bruce about this before they utilised the Gauntlet, conveying what he had learned on Vormir to the rest of the squad. One of Thanos' other children is the focus of the final piece of evidence.
The remaining Guardians and Avengers heroes search find Thanos on a world known as the Garden at the start of Avengers: Endgame, a place Nebula stated was the constant destination her father always discussed once his plan was complete.Their mission is to recover the Stones so they can utilise them to return everyone. Thanos tells them that he shattered the Stones after discovering him and securing him. The enormity of what he's saying is too much for them to comprehend at the time. If Thanos is telling the truth, they will have to live with the consequences of their failure, and everyone will die. When challenged, Nebula asserts that, while her father is many things, one of them is not a liar.
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Thanos had tortured Nebula her entire existence, and while she still harboured some twisted feelings for him, speaking falsely on his side would benefit neither him nor her. Thanos was as good as dead if there was no more possibility to get the Stones, which Thor saw to quickly. The essential element of this understanding, however, is that since Thanos always kept his word, he most likely did not perform the Snap on Tony because he had promised Strange that he would spare Tony's life. That sliver of honour turned out to be his downfall.
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Thanos would have obtained the Time Stone eventually, even if it took time and a lot of dead; otherwise, there wouldn't have been 14 million scenarios where they lost in Infinity War. Regardless of how he went about it, he needed to kill Tony before Strange intervened, because Tony was the only intellect capable of time travel in an afternoon. Even if he didn't murder him at the time, there was a 50/50 possibility that Tony would die in the Snap, and then there were all the other variables to consider, including other deaths that heightened or diminished Thanos' danger. The Illuminati of Earth-838 had other options after defeating Thanos with the Book of Vishanti, or perhaps that was an option after 15 million possible casualties. However, one thing is certain. Thanos most certainly saved Tony, but by doing so, by disturbing the religion of chaos that he had always preached, Thanos doomed himself, and Strange was the mastermind behind it all.