While most Marvel events happen on Earth, there is an entire universe of heroes and villains battling it out on other planets in the comics. As Marvel Cinematic Universe movies like Guardians of the Galaxy, Avengers: Infinity War, and the upcoming Eternals prove, the further off Earth one goes, the greater the chance for intergalactic cosmic war.
Cosmic wars between alien races in Marvel Comics often pull in Earth's mightiest heroes, and they see firsthand how destructive and devastating these wars in the far reaches of space are. Although some cosmic wars have had a major impact on Earth, there have also been some that were more devastating than others.
The Kree-Skrull War
The Kree-Skrull War was the first major cosmic war that took place in Marvel Comics. The war in the series played out in The Avengers #80-97 in 1963. The Skrulls showed up before this in The Fantastic Four and The Kree had already sent Captain Marvel to Earth as a spy, although she turned good and helped Earth.
What mattered here was that the Kree and Skrull alien races had warred for eons and even these issues did not end the conflict, as they continued at war until just recently when Hulkling, whose mother was a Kree princess and his father, Captain Marvel, united the two alien races in the Kree-Skrull Alliance.
Chaos War
The crossover series Chaos War brought together some of the most powerful gods in the Marvel Universe to battle one common enemy, The Chaos King, Amatsu Mikaboshi. The war started after Siege, where Norman Osborn led a strike that destroyed Asgard. This is when The Chaos King attacked and it devastated the Marvel Universe.
By killing the villain Nightmare, The Chaos King struck out at Hercules' allies; even the dead rose when Hades released them to battle the villain. What resulted was The God Squad forming, composed of some of Marvel's mightiest and most powerful characters, but even they lost. Hercules ended up giving up his godhood to beat The Chaos King and returned the cosmic Marvel Universe to a safe place.
War Of The Kings
War of the Kings saw a villain known as Vulcan return to try to conquer other nations by taking out their kings. Vulcan was a former X-Men member and the brother of Cyclops; no one remembered him because Professor X wiped his existence from everyone's minds. Mad with power, Vulcan attacked The Inhumans, The Kree, and The Shi'Ar warriors in The Imperial Guard.
The event was extremely destructive as Vulcan's attack almost destroyed the entire cosmic universe. It all ended when Black Bolt took the battle to Vulcan and seemingly sacrificed his own life to stop the villain once and for all. It also ripped open a hole in the galaxy that brought in a new threat known as The Fault.
King In Black
King in Black was the most recent cosmic war in Marvel Comics. This battle saw Knull, one of Marvel's most powerful cosmic characters, setting off to take over every known world, engulfing them in darkness and enslaving all living beings in symbiotes that he mentally controlled.
This battle centered on Earth, where most of Earth's heroes were captured and turned into symbiotic monsters. However, it stretched into the cosmos as well, with the Guardians of the Galaxy, Nova Corps, and more fighting. While Venom saved Earth, there were several alien races that Knull wiped out completely before he fell, destroying many worlds.
Annihilation Conquest
The Annihilation Conquest came after the first Annihilation, which itself was the most destructive cosmic war in Marvel history. This war started with the galaxy already devastated and so many lives already lost. Nova was on a mission while Xandar remained in ruins and he ended up in another giant battle.
This saw the Phalanx controlled by Ultron attacking the Kree homeworld and Nova was there to aid the Kree in this battle with the newly resurrected Star-Lord. While this was not as destructive as other wars, it is important because this is where the Guardians of the Galaxy that MCU fans know was born.
War Of The Realms
War of the Realms saw Malekith, the Dark Elf, and his army attack all the realms, attempting to conquer every world. While MCU fans saw Malekith in Thor: The Dark World, the version here was more powerful and decimated several worlds.
This cosmic Marvel war began when nine of the 10 Realms fell to Malekith, including Asgard. There was only one realm left to conquer and that was Midgard, also known as Earth. Malekith attacked and Earth began to burn and countless people died. The Asgardians came to Midgard's aid and finally won, with Thor becoming the new All-Father.
Secret Wars (2015)
Secret Wars was more than just a basic cosmic war. It was an event that destroyed every world in the multiverse, with Earth-616 and the Ultimate Marvel Universe the final two before they crashed and resulted in nothingness. Almost every character in existence died outside of a small handful, who all appeared after a new world formed.
This was Doctor Doom's Battleworld, where he was god, and every character existing there served him or was banished to survive in a dangerous landscape. In the end, the multiverse was restored
The Thanos Imperative
The Thanos Imperative began while the survivors of War of the Kings dealt with The Fault that arose following that war. Dark forces from that rip in the galaxy where death did not exist began to slip into this one and they began to conquer worlds.
The cosmic Marvel characters teamed together, with Silver Surfer, Quasar, and more battling to save the universe. However, they were not enough until the Guardians of the Galaxy brought in Thanos to help them. This was a highly destructive war, with the Nova Corps dealt a devastating blow, losing countless warriors.
The Infinity Trilogy
MCU fans saw a part of the Infinity storyline from Marvel Comics in the movies. However, the version in the comics was bigger, deadlier, and caused even more mass destruction. Just like in the movies, Thanos set out to find the Infinity Gauntlet and wiped out half the universe as he tried to win over the love of Lady Death.
That was only the start. Adam Warlock, a cosmic character who has still not appeared in Marvel movies, stopped him in the comics and took the gauntlet to protect, but it corrupted him and he began to destroy worlds until the cosmic heroes brought in Thanos to help stop him. More people died in this than any other Marvel cosmic war, but most were returned to life in the end.
Annihilation
While there were not as many deaths as in the Infinity trilogy, Annihilation was worse. That is because the deaths and destruction in this war were all too real and they were not reversed with a snap of the finger. This war was so deadly that the Shi'ar set up a program to help soldiers deal with the Post Traumatic Stress Disorder almost all of them experienced.
Annihilus from the Negative Zone attacked in this war and he practically destroyed the entire Nova Corps, leaving Richard Ryder's Nova a broken man. Nova had to work with the Kree Empire, and eventually team up with Thanos and Galactus just to stop Annihilus. Annihilation caused countless deaths rocked the entire cosmic Marvel Universe, something many warriors never recovered from.