But the real battle was won in the hearts and minds of moviegoers. Iron Man, The Incredible Hulk, Captain America: The First Avenger, Thor, and Iron Man 2 were slickly produced popcorn entertainment that still felt like they had a big, beating heart at the center of their stories—an essential ingredient to creating affection for characters that would extend beyond the life cycle of the box office. Just as Marvel had done with its comics decades before, the company realized that if it could create beloved heroes in their own series of films, bringing them together in a single team-up would not only generate even more enthusiasm, but would serve as a payoff for fans who committed to seeing every movie the studio put out. Consider it the cinematic equivalent of the comics’ crossover stories, in which a cliffhanger in Uncanny X-Men would continue in an issue of The New Mutants, and so on. An even more accurate comparison would be the so-called “event” series, in which a limited-run book tells a self-contained story, yet possesses characters and subplots whose arcs play out across a variety of other titles. (One of the most noteworthy of the modern era: 1991’s Infinity Gauntlet.)

The Avengers was meant to be the event series around which all the other films rotated. Thor and Hulk will have plenty of adventures on their own, but in planning to cap off the MCU’s first “phase” with the Joss Whedon-directed team-up, Marvel was introducing the rich currency of a shared universe straight into the mainstream of Hollywood cinema. For the first time in most movie fans’ lives, a culturally ubiquitous and commercially popular variety of films were going to be linked together by a single movie. Even if it wasn’t wholly unprecedented, it certainly felt like it was. Iron Man, Hulk, Captain America, Thor, and (let’s be frank) supporting players Hawkeye and Black Widow would join forces for a story that had been carefully prepped from the moment Nick Fury told Tony Stark that he wasn’t the world’s only superhero.

It was exhilarating. A lot of that has to do with Whedon’s excellent writing and directing, which juggles more than a half-dozen characters and multiple story arcs with the breeziness of a Saturday morning serial. He’s patient with the introductions; it speaks to his intentions that Black Widow gets the same screen time and setup as the characters who’d already headlined their own films. Even when Thor flies into the middle of the action without so much as a “Hello, my name is…” it’s so that the Asgardian can take a few quiet minutes of one-on-one time with Loki—a reminder of the character’s emotional core and personal stakes. Each Avenger gets a hero’s welcome of sorts; even if you didn’t catch it in a crowded theater of vocal fans, the movie often rah-rahs for you—its love for these characters is palpable. And the love of all of them in one room? Something special.

That was the unique thrill of seeing these heroes interact: There was a sense of boundaries being broken. When a door opens in an Avengers film, anyone could walk through, be it a recurring presence like S.H.I.E.L.D. agent Phil Coulson or Loki himself. It was surprising, and fun, a world of possibilities coming together before your very eyes. Avengers assemble, indeed.

And once it was over, regardless of your level of enjoyment or how many times you re-watched it (and for some of us, both numbers were high), the Rubicon of the shared universe had been crossed. Not unlike the first time you ride a roller coaster versus the second time: still thrilling, but you know what to expect. Look no further than the opening moments of Avengers: Age Of Ultron for confirmation. Whedon doesn’t bother with any intros—he knows we know who these people are, and that we’re ready for their story to progress—so he dispenses with any preludes, dumping us straight into the action and giving a single shot of all of the Avengers leaping headlong into battle in the first minute.

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Screenshot: Avengers: Age Of Ultron

There’s no thrill of excitement at simply seeing these various characters all together. This is just how it’s supposed to be. It was what we already expected, only one film into the flagship franchise of the MCU. Just a couple of films later, Captain America: Civil War was pitting the entire array of heroes against one another, before Infinity War reduced half of them to dust, and Endgame brought them back to life. It’s still a kick to see unexpected pairings spring to life; Infinity War saw Dr. Strange and Iron Man fighting together on the streets of Manhattan, and Black Widow duking it out side-by-side with Okoye. In Endgame’s climactic fight, we see Hawkeye handing off the Infinity Gauntlet to Black Panther, while Captain Marvel gives a smile and an assist to Peter Parker.

But that’s all they are, from now on: fun pairings we expect to see, that we’ve seen before in other permutations, and now want to see more of. The genie of the shared universe has been fully released from the bottle, and no amount of mass team-ups (and Endgame has a truly staggering tonnage of heroes) can put it back and recapture that initial thrill of seeing it all come together. The MCU will continue to be a source of entertainment and fun, and new generations will discover its pleasures with an equal degree of giddy enthusiasm.

But in 2012, it really did feel like our world was invaded—not by the Chitauri, but by the colossus of the Marvel Cinematic Universe. For good and ill, Hollywood cinema hasn’t been the same since.