In Avengers: Infinity War, Doctor Strange doesn't seem to get nearly enough credit for how he engineered the one option out of millions to save the lives of half the Marvel Cinematic Universe. Thanos already had four of the six Infinity Stones he needed to complete the Gauntlet, sacrificing his daughter in the process. Determined now to see it done while still mourning, Thanos looked out on his homeworld and saw Tony Stark waiting for him. He confirmed his suspicions that one of his other children, Black Maw, was dead before speaking to his vision of the universe. Shortly thereafter, the melee begins, and their only real hope seems to be Peter Quill's plan. Amazingly, they can execute it to perfection until Quill realizes Gamora's fate. The plan would have worked, but instead of capitalizing on their advantage, Starlord squanders it by attacking Thanos, dislodging Mantis and her sedating effect from his brow, and the Mad Titan unleashes his full fury against the assembled combatants.
After tossing orbiting moons and shattering eldritch duplicates, Thanos stabs Stark in the side before clenching the Gauntlet to deliver the finishing blow. Strange calls out and tells Thanos that he will give him the Time Stone if he spares Tony's life. Thanos warns the mage that he will not countenance any betrayals, and an agreement is struck. Strange sends the Stone floating toward Thanos, and he promptly sets it into the nearly complete artifact of ruin and renewal. As Thanos departs to retrieve the sixth and final Stone, Stark asks Strange why he made that choice, and the answer is interesting. Given the totality of information in hindsight, Strange trapped Thanos in an honor bomb and waited for it to explode.
When Strange lived through the 14 million branching timelines of Infinity War, the only one he saw where Thanos was defeated was the one where Tony sacrificed himself using the Iron Gauntlet to turn Thanos and his forces to ashes. Strange wasn't asking for Tony's life to be spared out of any sense of camaraderie; he was trying to ensure that Tony would be around to save everyone, himself included. The only problem with his plan is that, according to Thanos, the beauty of his vision was that it was inherently random. He did not dedicate any thought to who lived and who died; he simply snapped his fingers and let the chaos of probability determine the grisly details. This means Strange considered two different possibilities. Either Tony had a 50/50 chance to survive the Snap, or he knew once Thanos took him up on his offer that Tony would survive.
If Strange had opted for the 50/50 option, those were still far better odds than one in 14 million. However, it is more likely based on the evidence that Strange knew Thanos would keep his word and spare Tony's life, which would mean that he would have had to make an exception when he executed the Snap, singling out Tony specifically and therefore breaking the beauty of his plan. It had already been established that the Gauntlet can allow for all manner of nuance. One of the conditions of Tony's involvement in the Time Heist was that when they recovered all six Stones from the various timelines, there would be no alteration of the past five years so that his daughter's life would be protected. This was obviously possible since that is exactly what Tony did when he performed his Snap.
Tony's was much more specific than Thanos' since he must have created a carve-out to kill only the Titan's forces and no one else. It was also established that one could single out one person in the casting, as the Hulk described when he tried to bring Natasha back. He failed, but it does not appear it was because the intricacy of one life was not possible to parse with the power of the Stones, but rather because the acquisition of the Soul Stone required a sacrifice that could not be erased. Clint warned Bruce about that before they ever used the Gauntlet, relaying to the team what he had been told on Vormir. The last piece of evidence centers around one of Thanos' other children.
At the beginning of Avengers: Endgame, the remaining heroes from the Guardians and the Avengers seek out Thanos on a world known as the Garden, a place Nebula said was the constant destination her father always discussed once his plan was complete. Their goal is to retrieve the Stones so that they can use them to bring everyone back. After finding him there and securing him, Thanos tells them that he destroyed the Stones. At that moment, the enormity of what he is saying is too vast for them to reconcile. If Thanos was telling the truth, it would mean that they would have to live with their failure and everyone would remain dead. When pressed, Nebula speaks up and says that while her father is many things, a liar is not one of them.
Nebula had been tortured her whole life by Thanos, and though she still held some warped affection for him, there was nothing for him or her to gain by speaking falsely on his behalf. If there was no more chance to acquire the Stones, then Thanos was as good as dead, which Thor saw to swiftly. The important part of this insight, however, is that if Thanos always kept his word, then when he performed the Snap, he likely did indeed exempt Tony from it specifically since he had told Strange that he would spare Tony's life. That bit of honor proved to be his undoing.
It may have taken time and a lot of death, but at some point, Thanos would have gotten the Time Stone anyway; otherwise, there wouldn't have been 14 million futures where they lost in Infinity War. However he went about it, he needed to kill Tony as he intended before Strange stopped him from doing so since Tony was the only mind capable of creating time travel in an afternoon. Even if he didn't kill him then, there would've been a 50/50 chance that Tony died in the Snap, and then all the variables that would've applied after that, with other combinations of deaths that heightened or lessened the threat to Thanos. There were other means available to the Illuminati of Earth-838, who defeated their Thanos with the Book of Vishanti, or perhaps that was an option after 15 million potential losses. But one thing is clear. Thanos more than likely saved Tony, and in doing so, in disrupting the gospel of chaos he had always espoused, Thanos doomed himself, and Strange was the architect of the whole thing.