Phase 4 of the Marvel Cinematic Universe has introduced a cavalcade of new threats. This first section of The Multiverse Saga has introduced our favorite heroes to problems they can barely begin to comprehend. Not only do they have to contend with Variants of themselves across multiple universes and timelines, they have to deal with the repercussions that this brings. So many characters jumping across the multiverse has begun to destabilize different realities, which we learn in Doctor Strange in the Multiverse of Madness risks all universes undergoing an Incursion. But what even is an incursion? Can they be stopped? Why haven't they been a problem until now? All of your burning questions will be answered below as the MCU gears up to Avengers: Secret Wars.
When Do We First Learn about Incursions?
When Doctor Strange (Benedict Cumberbatch) enters Earth-838 with America Chavez (Xochitl Gomez), he is almost immediately captured and interrogated by The Illuminati, a group of superhero team leaders. One of the leaders, Reed Richards (John Krasinski), tells Strange that his very presence in the universe risks the safety of it. The more impact he makes on Earth-838, the more it can cause a butterfly effect that can set off an incursion. In Reed's words, "An incursion occurs when the boundary between two universes erodes, and they collide, destroying one or both entirely."
Strange doesn't seem too concerned about this problem because he was more worried about Wanda Maximoff (Elizabeth Olsen) at the time, but incursions are a very real threat. Doctor Strange variants are often the ones that cause them too.
Has an Incursion Already Happened?
So far, we have seen two incursions occur in the MCU and both were the fault of Doctor Strange. In What If...? A heartbroken Doctor Strange destroys his timeline by constantly resetting it to attempt saving the life of his girlfriend, Dr. Christine Palmer (Rachel McAdams) which he can never succeed at. His constant breaking of the timeline causes the timelines to crash into each other, where two Doctor Strange's fight for the survival of their own. Much like an incursion, when collided only one of these timelines can be saved, or none of them do. The artists on the show even visualize the colliding timelines in the same way an incursion is visualized in Multiverse of Madness. It's like the universes are disappearing and turning into black ink.
This incursion that 616-Strange witnesses was also caused by a variant of his, Sinister Strange. It isn't fully elaborated on, but it is known this incursion was caused by Sinister Strange who dreamwalked into other universes and was trying to save Christine. Dreamwalking is the process of possessing the body of your own Variant in another universe. At this point, it might seem like incursions only happen when Strange isn't able to handle a break-up, but that's not always true.
What Causes Incursions?
What triggers an incursion is sort of vague. It depends on each story, but it seems to happen whenever people break some sort of unwritten law of the universe. We see 616-Strange trigger one in Multiverse of Madness by dreamwalking into a dead body. Puppetting his own dead body caused some demons to go after Strange, but he used his magic against them and turned them into a cape. This disregard of these mythic rules is what makes Strange's longtime comic love interest, Clea (Charlize Theron) seek out Strange to stop an incursion, implying that these actions of his caused it.
In general, incursions happen whenever some kind of paradox occurs at the same point in two different universes. This could be something like an action that completely breaks the timeline, like when What If Strange kept trying to save Dr. Palmer's life. Alternatively, they can happen whenever someone travels between universes just to cause damage, which is what made Wanda such a threat in Multiverse of Madness.
Why Have Incursions Entered the MCU Now?
The one reason incursions haven't been a threat to Earth-616 up until now is because of a variant of Kang the Conquerer, named He Who Remains (Jonathan Majors). This Kang variant was the sole survivor of a previous Multiversal War, fought by an infinite amount of himself. At the end of the war there was only He Who Remains, who decided to close off his own universe from the rest of multiverse, that he dubbed The Sacred Timeline. In the show Loki, we see that this Kang has created his own authoritarian task force called the Time Variance Authority to patrol the timeline and remove any possibilities of the timeline spitting into new universe. The TVA did this by sending any variants who deviate from Kang's path to a place called the Void.
But after an eternity of managing the universe, he grew old and tired. So he led two variants of 616-Loki to his Citadel at the End of Time. It was here where one of the variants, Sylvie (Sophia Di Martino) killed He Who Remains, which caused his Sacred Timeline to start connecting to the wider multiverse.
Now that the 616 Universe isn't separated from the rest, it has allowed Doctor Strange to screw up a spell that allows every other Sony Spider-Man film to crossover with the MCU. It's why we are now witnessing other universes that are similar to 616 in What If...? It's also the reason why America Chavez was only now able to slip into the MCU in Multiverse of Madness. But besides it giving the MCU a story reason on why it can now crossover with any past Marvel adaptation, Sylvie killing He Who Remains is also what can allow incursions to happen in 616.
What's Does This All Mean?
Incursions in the comics are the biggest events which caused the 2015 version of Secret Wars written by Jonathan Hickman. This is when main comic universe 616, and the Ultimate universe 1610, collided into one. The incursion destroyed both worlds. In the wake of their destruction, Doctor Doom obtained the powers of the aliens known as the Beyonders to recreate a new universe in his image called Battleworld that was an amalgamation of many universes. In the end, the Avengers and Reed Richards beat Doctor Doom and returned the 616 Universe back to reality, only now it includes the characters from the Ultimate universe that fans actually liked. This is why Peter Parker and Miles Morales now exist together in the main comic timeline.
This is all to say that all of these incursions and multiversal shenanigans will be a big factor in the upcoming Avengers movies. The multiverse is about to be become something much scarier than an excuse to bring in more cameos.