One of the most exciting things about the Captain Marvel movie last year was the long-awaited introduction of the Skrulls into the MCU. While the Kree had shown up earlier in the Guardians of the Galaxy franchise, the official arrival of the Skrulls meant the films may get to one of the big pillars of Marvel Comics history: The Kree-Skull War.

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Before the films get deep into cosmic warfare, the comics themselves are revisiting the subject in the crossover event Empyre. With that in mind, let's take a look at some important things to know about the event that started it all.

10 The First Real Crossover

Kree-Skrull-War

Marvel distinguished itself in the '60s by creating a tightly connected shared universe where it wasn't unusual for Spider-Man to bump into the Fantastic Four. They pretty much all lived in the same city anyways.

But the Kree-Skrull War was in many ways the publisher's first true crossover event, entangling characters from across Marvel Comics. The Avengers, Inhumans, and Fantastic Four all played a role, as did Mar-Vell, the original Captain Marvel. Spanning nine issues in The Mighty Avengers, the saga set the template for major crossovers like Secret Wars in the '80s.

9 It's An Allegory For The Red Scare

Skrulls feature

The most fascinating – and threatening – thing about the Skrulls is their ability to assume the form of pretty much anyone they come into contact with. They could be anyone, anywhere, and this paranoia drove a lot of the story in the arc, running issues #89-97 in The Mighty Avengers.

Writer Roy Thomas introduced a senator who held hearings to determine to what extent Skrulls had infiltrated American society, a not so subtle nod to the Red Scare hearings of Senator Joe McCarthy, who used his bully pulpit to create fear about possible communist infiltrations in '50s America.

8 Rick Jones Saved The Universe Because Captain America Was Too Much

Fans of the comics will know that Rick Jones plays a decisive role in the Kree-Skull conflict by using the Destiny Force, but how he got to the point might be less understood. Rick got entangled in the war due to his connection to Mar-Vell, as they were bonded through the Nega-Bands at the time, meaning only one was on Earth while the other was in the Negative Zone (not complicated at all).

As for why Jones was with Mar-Vell at all, it had to do with his former partner, Captain America. Rick had assumed the name and costume of Cap's (once) dead friend Bucky, which really wasn't the best idea. Things got frosty, prompting Rick to go looking for a new partner.

7 Vision & Scarlet Witch

One of the most notable elements of the Kree-Skrull War storyline is the introduction of the romance between Vision and the Scarlet Witch. A complicated, to say the least, pairing that has seen many ups and downs over the years, this pivotal relationship has also been integral to the MCU films.

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Writer Roy Thomas developed the couple to help make the android Vision more human. The unique pairing of the powerful robot and shy mutant proved a hit, and the two later got married and had children. Vision later dies and comes back, and Wanda eliminates reality and most mutants, so it's your typical relationship.

6 Inhuman Connection

Though the Kree-Skull War took place within the pages of the Avengers monthly title in 1971, it wasn't entirely an Avengers affair. As mentioned earlier, the Fantastic Four took part and so did the Inhumans.

The Inhumans played a relatively small role in the story, but their entire existence owes a debt to the Kree-Skrull conflict. The war waged for millions of years. At some point, probably for the jokes, the Kree built an outpost on Uranus. From there, they experimented with ancient humans, elevating some of them into the Inhumans for the purpose of creating an army to fight the Skrulls.

5 The Supreme Intelligence

Those experiments on early humans only ended because of a prophecy the Kree feared would lead to the destruction of the Kree Supreme Intelligence. The Supreme Intelligence, also known as The Supremor, was the result of an arms race that started when the Skrulls built the Cosmic Cube.

The supercomputer played a huge role in the outcome of the Kree-Skrull War. Together with Mar-Vell, the Supreme Intelligence helped unlock the Destiny Force within Rick Jones to bring about the end of the conflict.

4 This Island Earth

Roy Thomas was inspired by history when it came to crafting the story, but one of the main inspirations for the Kree-Skrull War was a novel called This Island Earth.

A science-fiction novel written by Raymond F. Jones and published in 1952, the book tells the story of a race of aliens, the Llanna, who are engaged in a millennia-spanning intergalactic war with another race, the Guarra. The Llanna recruit humanity for a group called "Peace Engineers" and seek to use Earth as a pawn in their war. Sounds familiar.

3 They Didn't Collect It For Thirty Years!

Unthinkable today in the age of the trade paperback, Marvel didn't collect the historic nine-issue run of the Kree-Skrull war for nearly thirty years. The first collected edition debuted in 2000, meaning those who wanted the whole story during the '80s or '90s had to dive in back issue boxes.

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Collecting the story certainly contributed to its enduring popularity, and came at the right time as comic book movies really ramped up in quality (and quantity) in the early aughts. Re-reading it now would be a good way to get ready for Empyre, though there is a primer out there.

2 The Accuser

Ronan the Accuser with his hammer.

Another aspect of the story that reflects the overarching theme of paranoia is Ronan the Accuser's plot to overthrow the Supreme Intelligence. Although The Supremor is a computer, it is also installed as the leader of the Kree race.

Ronan isn't down with being ruled by a machine, even if it's made up of millions of Kree minds, so he plots to overthrow the machine. Good thing he failed, though, as had the Supreme Intelligence fallen, Rick Jones would not have unlocked those sweet Destiny Powers and the entire galaxy would have gone up in flames.

1 A Glimpse of the Future

Ms Marvel Carol Danvers Captain Marvel

Despite the fact that the Captain Marvel at the center of the Kree-Skull War is the Kree hero Mar-Vell, Carol Danvers, who ultimately inherits the title, also plays a small role. Just one of the many things changed in the Captain Marvel film, Carol is in her Ms. Marvel guise at this point in time.

Well, someone is. It turns out the Carol Danvers in the Kree-Skull War is actually a Skrull in disguise, just one of many, prefiguring decades of Skrull attempts to infiltrate and conqueror the planet Earth.

NEXT: MCU: 10 Details About Captain Marvel That You Missed In Endgame