There are a few really enduring images from Avengers: Infinity War that persist even well after the film has finished. Gamora's lifeless body, Peter Parker's desperate pleas not to "go", Rocket Raccoon broken by the second death of Groot... And pretty much all of Robert Downey Jr's excellent performance as Tony Stark reacting to the defeat he assumes has happened is possibly the most captivating.
Downey Jr is a towering presence in the movie and it might well be his best performance as Stark. He's particularly brilliant when Doctor Strange barters for his life by giving the Time Stone up to Thanos to spare him with that desperate, defeated why question punching straight through the audience's collective hearts.
Obviously, as has been well-covered everywhere now, Strange's response is "we are in the end game now" - an iconic line so impactful that it was assumed to tease the real Avengers 4 title. But it was no consolation to Stark to know that Strange had a plan that had to involve him not dying and that had to include Thanos initially "winning". It was Strange's "betrayal" that really took the wind out of Stark - which is why the Doctor felt so compelled to apologise to him as he faded away.
And really, Stark should have been able to forgive Strange, because it was he who probably inspired his extreme measures approach to defeating Thanos. Strange's "plan" is borne out of necessity - he knows that something drastic has to be done to ensure victory in the long-term and that the price might seem grave, but that it is entirely justified by the perceived outcome. Which has been precisely Tony Stark's driving motivation ever since he put the Iron Man armour on.
As his actions have proven over the past few movies (and in Age Of Ultron onwards in particular), he is a "bigger picture" thinker who looks at the end goal even at the expense of everything else. He is willing to put his fellow Avengers under house arrest or limit their personal freedoms in order to ensure their survival. He is willing to create a super robot to fight another super robot he unwittingly unleashed on the world. He is willing to kill himself if it means nobody else has to die.
And most importantly, Stark has used the exact same terminology Strange used to justify the cost of victory. Back in Age Of Ultron, when Stark was confronted by the other Avengers for creating Ultron by over-reaching with his attempts to defend the Earth, he justified himself by saying this:
"A hostile alien army came charging in through a hole in space… we're standing 300 feet below it. We're the Avengers. We can bust arms dealers all the live-long day, but… that up there? That's the end game. How were you guys planning on beating that?"
The end game. Strange used Stark's own words against him to appeal to him, but to also attempt to hint at a positive outcome despite appearances. It's that sort of subtle attention to past detail that really makes the MCU so great.