Secret Invasion was the culmination of years of plotlines. Writer Brian Michael Bendis had begun building it in New Avengers #1 in 2005, crafting the story for three years before unleashing it with artist Leinil Yu. During this time, Bendis also created the Illuminati, a covert group of Marvel's greatest leaders — Iron Man, Namor, Black Bolt, Professor X, Doctor Strange, and Reed Richards.
The Illuminati worked behind the scenes, but they had a significant influence on the Skrull invasion. Things would have been very different for the Skrulls without the Illuminati. While the Illuminati had formed to prevent just such an attack, their past actions made the Skrull invasion possible in the first place.
10 The Return Of The Thunderbolts
The Thunderbolts aren't always the Avengers' best allies, but they played a significant role in Secret Invasion. The villains were working for the Superhero Initiative, completely under SHIELD's control. The Skrulls took the Avengers off the table early in the conflict, leaving the Thunderbolts to act as the tip of the spear.
However, without the Illuminati's Iron Man and Reed Richards, the Thunderbolts never would have existed. Iron Man and Reed had brainstormed the strategy for the pro-Registration heroes, coming up with the idea for the new Thunderbolts. Without them, the Skrulls would've done less damage.
9 They Never Looked Captain Mar-Vell's Return
In the months leading up to Secret Invasion, a surprising character returned to the land of the living. Mar-Vell was the original Captain Marvel and had died of cancer many years prior. Yet, he made a momentous return. The superhero community celebrated, but he had a terrible secret — he was a Skrull secret agent.
The Illuminati felt it was their job to look into things behind the scenes, but they never parsed out what was going on with Mar-Vell. As the first major league cosmic hero, it would make sense for everyone to be happy he was back. However, the Illuminati should've investigated his return more. It would have revealed the Skrulls' trickery sooner.
8 They Never Looked Into SHIELD's Operation In The Savage Land
During the New Avengers' first mission, they learned that a rogue element of SHIELD was doing something in the Savage Land. This seemed like a big deal, yet the Illuminati never looked into it. It was later revealed that this rogue group comprised Skrulls, who had taken almost complete control of SHIELD by this time.
Iron Man knew all about the operation in the Savage Land, but never truly looked into it. He easily could have mobilized the Illuminati — who had powers, experience, and equipment — then hunted down the truth. Through their ignorance, the Illuminati allowed the Skrulls free rein over the Savage Land.
7 Driving The New Avengers Underground
The Avengers are Earth's Mightiest Heroes, but they were splintered during Secret Invasion. There were two teams — the New Avengers and the Mighty Avengers. The New Avengers comprised unregistered heroes and were driven underground. The Mighty Avengers were the main team and hunted the New Avengers.
The Illuminati played a key role in holding the New Avengers down. The Illuminati's membership supported the Superhero Registration Act. Without that, the New Avengers would've had more resources to hunt down the Skrulls after they learned Elektra was a Skrull. Instead, the Avengers were pitted against each other, distracted from the Skrulls grand scheme.
6 The Skrulls Gained Their Power Copying Ability From The Illuminati
Marvel heroes have amazing powers. The sheer variety of them made it difficult for the Skrulls to replicate them. Skrulls with superpowers were very rare, and it's why they could never attempt a large-scale infiltration disguised as Earth's heroes. However, the Illuminati gave the Skrulls the tools they needed.
After the Kree-Skrull war, the Illuminati attacked the Skrull homeworld as a warning. As powerful as the group was, they got captured by the Skrulls and the aliens began experimenting on them. The Skrulls used what they learned during this time to begin research into replicating powers, which made their invasion possible.
5 Civil War
Civil War was disastrous. The battle between Captain America and Iron Man made the superhero community choose sides, something which played into the Skrulls' hands. The conflict did tremendous damage, sowing seeds of discord and distrust in the superhero community. The Illuminati were almost entirely pro-Registration, helping along this process.
The fracture in the superheroic community made the Skrulls' job much easier. No one trusted anyone, and no one wanted to work together. On top of that, Iron Man's victory put SHIELD in charge of administering the Superhero Initiative, which gave the Skrulls in control of SHIELD direct access to heroes.
4 Black Bolt Was A Skrull
Black Bolt's inability to speak comes in handy when playing his cards close to the vest. He joined the Illuminati as the representative for the Inhumans, relying on Professor X's telepathy to communicate his thoughts to the group. However, Black Bolt's secretive nature made it very easy for the Skrulls to replace him.
The aliens knew how to subvert Xavier's telepathy and Strange's magic. Having Black Bolt as a Skrull allowed the invaders to keep abreast of the Illuminati's plans. Black Bolt also kept the Illuminati from looking too deeply into the Skrull problem.
3 They Never Investigated Spider-Woman
Spider-Woman was revealed to be Veranke, the Skrull Queen. The Skrulls had captured Jessica Drew prior to her joining the New Avengers. While "Spider-Woman" was on the Avengers, there were many questions about her relationship to SHIELD, Hydra, and how she got her powers back. Unfortunately, everyone took her word for it.
The Illuminati should've investigated Spider-Woman's claims. She could've been working for Hydra, and yet everyone believed she was on the up and up. While Nick Fury vouched for her, that didn't guarantee that Spider-Woman hadn't sided with Hydra along the way. The Illuminati were supposed to look out for threats, yet missed a huge one.
2 They Got The Hulk Off-World
The Hulk's reputation is well-known throughout the universe and has kept many alien races from attacking the Earth. The Skrulls were just as frightened of the Hulk as everyone else was and were reticent to draw his ire. The Illuminati getting rid of Hulk was the best thing the Skrulls could've hoped for.
With the Hulk out of the picture, the Skrulls had things a lot easier. Even though Hulk returned by the time the Skrulls' attack began, he was on his own and no one trusted him. Hulk's grudge kept him from helping the heroes until things got drastic, giving the Skrulls all the time they needed to wreak havoc.
1 The Skrulls Wanted Revenge On The Illuminati
The Kree-Skrull War changed everything. Suddenly, Earth was thrust into intergalactic politics. The Illuminati decided they had to make a statement, so they went to the Skrull Homeworld and told the Emperor to leave the Earth alone. He responded by having them captured and experimenting on them.
The Illuminati broke free and wrought havoc on the Skrulls. The Skrull Emperor vowed revenge. Even after the destruction of the Skrull Homeworld and the Emperor, the remnants of the Skrulls wanted vengeance against the Illuminati for embarrassing them. This hatred fueled their ambition and led them to attack during Secret Invasion.