Marvel's Civil War was a cataclysmic event and began the tradition of Marvel's summer event in earnest. It pitted the two heads of the Avengers, Captain America and Iron Man, against one another over the government-mandated Superhero Registration Act. The fight, its casualties, and its fallout transformed Marvel Comics for years, and the shockwaves of the story, its ideas, and its reception have shaped the comics to this day.
However, the follow-up, Civil War II, didn't have quite the same titanic effect. An underwhelming story and event-fatigue left audiences far less enthralled, and it tried to shift Marvel Comics into a far less appealing direction. It pitted Captain Marvel against Iron Man over what to do with a clairvoyant Inhuman called Ulysses, and the story's ending didn't quite have the same titanic impact.
10 CIVIL WAR: The Annual Summer Event
While Avengers Disassembled and House of M proceeded Civil War, it was the first true annual blockbuster summer event for Marvel.
It left an impact, made real-world news headlines, and everyone was dying to read the next issue. Secret Invasion, Siege, Fear Itself, AvX, and every one that has followed tried to recreate the smash success of Civil War with varying success.
9 CIVIL WAR II: Death Of The Hulk
Civil War II took Bruce Banner off the table. Amadeus Cho was the Totally Awesome Hulk at this point, and Bruce Banner had become a secluded scientist. However, Ulysses had a vision of the Hulk killing all the Avengers, so everyone went out to confront Bruce.
In a moment of tension and panic, Hawkeye used a gamma-powered arrow to kill Bruce Banner on the spot. This stuck through to Avengers: No Surrender, and, thankfully, Al Ewing and Joe Bennett have brought back the Hulk in full with their Immortal Hulk series.
8 CIVIL WAR: Captain America And Iron Man
Captain America and Iron Man had their differences and butted heads well before Civil War, but this story solidified it more than any story before. These two had come to truly resent one another. Cap was an idealist unwilling to allow super-powered people to be made into government soldiers, and Tony was the pragmatist who believed that they need to do this so worse things don't follow.
It brought to the surface every difference the two ever had, and it escalated even further with the exposure with the Illuminati and the mortal struggle as the world ended in Time Runs Out.
7 CIVIL WAR II: Iron Man The Martyr
Civil War II tried to reposition Iron Man as the upbeat and well-meaning centerpiece of the Marvel Universe, only to kill him/put him in a coma. This was during the peak of Marvel replacing its headliners with the likes of Sam Wilson, Jane Foster, and Riri Williams.
Whatever one may think of that, this step in that process tried to completely redefine what Iron Man had been in the Marvel Universe. His semi-authoritarian technocratic tendencies were gone; Spider-Man in power armor stood in his place. His "death" was meant to help usher in the next generation of idealists, but his character just wasn't convincing in this role.
6 CIVIL WAR: S.H.I.E.L.D. Authoritarianism
S.H.I.E.L.D. had always had a looming presence in the Marvel Universe, but this event made them a centerpiece of everything. They were always there, looking for unregistered heroes to arrest.
They were the top police force in the country, and this total authoritarianism was echoed in Norman Osborn's H.A.M.M.E.R., Cap's S.H.I.E.L.D. presence during Time Runs Out, and the S.H.I.E.L.D. of Captain America: Winter Soldier.
5 CIVIL WAR II: Death Of War Machine
War Machine was another casualty of Civil War II, and Rhodey certainly deserved a better fate than being abruptly killed by Thanos.
He was another one who stayed in the grave for some time, and he didn't get a return until after Tony was resurrected too in Iron Man #600.
4 CIVIL WAR: Luke Cage, The Avenger
Luke Cage had already been recruited into the New Avengers team a couple of years before, but his presence in Civil War led him to being a centerpiece of the Avengers for years to follow. He led the New Avengers when they went underground during the Initiative era, and it skyrocketed his popularity.
It continued through into Marvel NOW!, where Luke led the Mighty Avengers team, which formed to fight Thanos' forces during Infinity.
3 CIVIL WAR II: War Without Cause
Civil War II had an exceedingly thin premise. Ulysses could see the future. Captain Marvel wanted to use it to preemptively arrest criminals, and Iron Man wanted to completely ignore Ulysses' abilities.
This could have been easily discussed, and there are dozen of compromises to be reached between the two points. No one is forcing them to fight beyond narrative contrivance.
2 CIVIL WAR: Death Of Bill Foster
Bill Foster took up the mantle of Goliath in the mid-70s. He never gained a lot of popularity, but he popped up a few times in Marvel Two-in-One and Defenders in the interim between then and Civil War. He finally came back into prominence as a central figure on Cap's side of things in Civil War.
He was killed by a cyborg clone of Thor called Ragnarok. His death is one of the few Marvel deaths that have stuck, though his nephew, Tom, became the new Goliath and popped up a couple of times in Avengers and War of the Realms: Giant-Man since then.
1 CIVIL WAR II: Grounded
Civil War II also represented a turning point for the Guardians of the Galaxy. The Vision shot down their ship during a battle, and the Guardians were stuck on Earth for a while afterward.
It helped splinter the team, and Guardians of the Galaxy and affiliate books came to represent a bit of a sitcom for a good few months in the aftermath.