"Tony, there was no other way."
Doctor Strange knows there’s only one scenario out of 14 million, six hundred and five possible future outcomes where the Avengers defeat Thanos. So the moment he traded the Time Stone for Tony Stark's life, after specifically telling him he would never do that, it was clear letting the Mad Titan “win” to keep Iron Man alive was the only chance they had at saving the universe.
But beyond deciphering the big "what" of his grand plan in Infinity War, there are other pressing questions concerning the specifics of the plan—for instance, how Strange had them fight as hard as they could even though he already knew the outcome. Why, if things were just going to end with him giving Thanos the Time Stone anyway, did he fully commit to trying to beat Thanos right there?
Doctor Strange knew the battle was doomed even before Thanos showed up on Titan; he foresaw Quill would lose his temper and screw everything up. Yet the master of the mystic arts still fought relentlessly, and he let the rest of the team take a beating—including Tony, who got a blade through the stomach. But if their effort was doomed, why go through all of that instead of just fastforwarding to the ending, which is presumably the most important part of his master plan?
For starters, it seems that how they lost was just as important as the fact that they lost, since it was the only way to teach Tony Stark the lessons he need to undo Thanos' snap. But knowing them might cost Tony everything.
It was obviously necessary for Thanos to walk away believing he won. That way, he wouldn't realize the fight wasn't over. And since Tony is clearly a major (maybe "the" major) key to ultimate victory, losing the fight was just as important for him, because it had necessary lessons to teach him.
The first lesson: Despite catching Thanos by surprise with a smart plan and a team of powerful heroes, they couldn't physically defeat him, especially while he possessed so many Infinity Stones. (And they definitely couldn't have. Thor put the universe's strongest weapon into his chest and Thanos lived, and Ronan was nearly indestructible thanks to just the Power Stone.) No one fought as hard or as bravely as Tony did; he literally threw everything he had at Thanos. And what did it get him? A single drop of blood. The failure to beat him physically showed him that to beat the Mad Titan the Avengers will either have to use guerrilla tactics or wield Infinity Stones themselves.
Tony also saw what happened when Star-Lord lost control of his emotions, but Tony already knows how damaging that can be since he did the same thing at the end of Captain America: Civil War when he learned Bucky killed his parents. But why Star-Lord flipped out—because Thanos sacrificed Gamora to obtain the Soul Stone on Vormir—is new, vitally important information for Tony. To undo something as grand as Thanos' great dusting, the Avengers will likely need to amass the Infinity Stones themselves. Now Tony realizes the horrible truth about what it takes to actually do that.
But Tony possibly having to sacrifice someone he loves to undo the "snap" isn’t the only horrible lesson he might have learned on Titan; it’s possible Doctor Strange didn't break his original vow to protect the Time Stone.
As he told Tony on Ebony Maw's ship: "But you have to understand, if it comes to saving you, or the kid, or the Time Stone, I will not hesitate to let either of you die. I can't, because the universe depends on it." And while it seemed like Strange undercut this, he may well have seen that the universe's fate depended on Tony surviving to fight another day.
And what did surviving mean? It meant watching Peter vanish in his arms, to know he failed the universe, to know how much pain and suffering took place so he could live. Tony knows better than anyone what's at stake now, and that it all happened so he could live. That's a terrible burden to bear, but Infinity War made it clear that "knowledge" is often accompanied by pain.
Red Skull told Thanos that it was his "curse to know all who journey here," and then Thanos echoed that sentiment when he told Tony "you're not the only one cursed with knowledge." Even the Soul Stone, which exacts a heavy price to possess it, is said to have "a certain wisdom." Knowledge comes with a terrible price, and for Tony that could mean paying the ultimate price.
Before they fought, Thanos told Doctor Strange, "The hardest choices require the strongest wills," and he was right. The heroes who try to undo the great dusting will surely have to make difficult choices and unthinkable sacrifices.
And thanks to Doctor Strange fighting as hard as he could despite knowing what would happen, Tony learned that he can’t match Thanos in a fight. He learned where he has to go and what he has to do to get the Soul Stone. And most importantly he learned failure is not an option, and that if the universe depends on it, one life is a small price to pay to stop Thanos from winning.
Even if that means giving up his own.
Mere mortals can't wield Infinity Stones the way a powerful being like Thanos can. If someone has to snap their fingers to undo what he did it will most certainly cost them their lives—a fact they'd surely know before making that decision.
Knowledge is a curse, and thanks to the lessons Doctor Strange taught him on Titan, nobody knows that better than Tony Stark.
What do you think? Why did the events of Titan have to take place exactly the way they did? Tell us in the comments below.