Both the Avengers and the Guardians of the Galaxy have faced genocidal zealots, cosmic Gods, and vengeance consumed warriors, and we as the audience have watched alongside them. So after all the victories, close calls, and the occasional genuine sacrifice, why does Infinity War work?
After all, we all knew that this film had to change the status quo of the MCU. A lot of us were expecting, even (to an extent) hoping for loss, death, and a fear of the villain, the now acclaimed "Mad Titan" Thanos. If that's the case, how come so many were left in shock and tears as the credits rolled? Having characters (particularly main ones) die in any number of ways is always sad, but Infinity War has made more out of it than a film of its ambition and requirements has any right to. So again, why does it work?
The answer, and arguably the greatest success of the both the Russo brothers and the screenwriters Markus and McFeely, is simply the crushing failure of the heroes. For there to be true despair, there must first be hope is a common saying, and it is applied brilliantly in this film. The heroes leap on every opportunity, every desperate plan and heart-breaking ultimatum in a frantic race to stop the universal annihilation Thanos will bring. And at every turn, they fail.
This is first shown by the progression of each of the characters in both previous films and between them. As the MCU has evolved, so have our heroes and their abilities, but here they mean nothing.
Iron Man has constantly upgraded and experimented with his suits to find the best combination, and here he seems to have found the perfect mix, unleashing his most powerful and versatile armour yet, only for it to be torn apart by Thanos' strength alone. Thor, on the other hand, realised his full potential near the end of the Ragnarok, then uses it to further empower the most powerful Asgardian weapon ever created, but when he finally has the chance, he can't kill Thanos fast enough to stop him unleashing the full power of the Infinity Stones against the universe. We've seen both Doctor Strange and Captain America grow more powerful with each film they star in, and when they each attempt to fight Thanos, they last literal seconds before being easily overwhelmed. And the list goes on.
Secondly, as mentioned earlier, both groups are pushed to see how far they'll go to survive, only for the sacrifices to mean nothing. Gamora makes her lover Star-Lord agree to kill her to stop Thanos from finding the Soul Stone. What happens? When Peter makes good on his promise, Thanos turns his blaster bolts into bubbles, takes Gamora, and kills her. Vision pleads for his lover, Scarlet Witch, to destroy the Mind Stone and therefore him so Thanos can't get it. What happens? When she does it, Thanos already has the Time Stone, winds the clock back a minute, and takes it without effort. Whenever the heroes think they've saved the universe despite the cost, it turns out they've done nothing.
Nothing the heroes do matters. The Guardians teaming up with Iron Man, Spider-Man, and Strange does nothing. Thor almost dying to make the war-hammer Stormbreaker delays Thanos for mere moments and nothing else. Having both the most technologically advanced country and its army to defend Vision means nothing when they are overwhelmed by the berserk Outrider army, just as how the combined efforts of Captain America, Black Panther, Black Widow, War Machine and the other heroes can't even slow Thanos down, even after we've seen how powerful and skilled each has become.
Yet another layer that adds to the despair are the well-established heroes and their stories, particularly their relationships, as Thanos' quest inevitably tears them apart.
Thor has been struggling both with his obligations to Asgard and his rivalry with his brother Loki for years. The end of Thor: Ragnarok, he's the King of Asgard and has reconciled with Loki, and the first scene of Infinity War has him witness his people slaughtered and Loki being killed by Thanos himself in vain attempt to save Thor.
The sisterly love that Gamora and Nebula finally found at the end of the second Guardians film is made into a tragedy, as Thanos tortures Nebula to force Gamora into revealing the location of the Soul Stone, and Nebula is left to struggle against Thanos to avenge her sister's death.
Tony and Steve have formed strong bonds to Peter Parker and Bucky Barnes respectively, only to collapse and watch them literally crumble into nothing. The creative team takes the most beloved relationships of the fans and kills them one at a time, and the survivors are left to mourn how all their struggles and growth only led to death and loss.
This goes even further than the romantic, paternal, and brotherly bonds shown too. The Avengers were torn apart in Civil War, so the obvious conclusion would be their reunion being the catalyst for Thanos losing. But instead, when Banner calls the renegade members, their actions once again simply delay Thanos' victory. To add to this, Tony's worst nightmare, the fear that created Ultron and had him at odds with Captain America for years came true, and he couldn't stop it. His desperation to protect those he cared about is rubbed in his face as his student clings to him begging to live.
The final nail in the coffin is the (no pun intended) hammering in of their defeat. When the heroes fight, they don't just fail, they fail hard. When the Hulk is beaten, you can see the blood and fear on his face, and you can really feel the vicious blows Thanos reigns down on him. Loki is strangled, has his neck snapped, and his lifeless body dropped at Thor's feet. Gamora's body is broken and bleeding when Thanos throws her off a cliff. Vision has his head torn apart for Thanos to get the last Stone, and the heroes are left beaten, bleeding and in horror as they watch those they love die in the Mad Titan's wake.
And what does Thanos do? He rests on a farm and watches a peaceful sunrise. He doesn't gloat, or mock, nor does he move on to the next stage of his plan. Why bring this up? Because Thanos achieved exactly what he wanted. The heroes can't jump up, mourn their losses, and resolve to prevent Thanos hurting anyone else. There's nothing to fight for, and the fact that Thanos is an actually respectable and understandable villain is what seals the pain felt, because we have to understand him to be wrong from a logical and thought out standpoint, not just by saying "oh he's the villain," and he wins nonetheless.
Avengers: Infinity War did what everyone and no one wanted. It took viewers and characters out of their comfort zones, faced them up against a truly intimidating threat, and destroyed them. And it made extra sure to take us along for the ride. Which is also what makes it a brilliant film, and a fine nexus point for the MCU.