Marvel has published many events over the years, but Civil War remains perhaps the biggest and most popular of all time. When I was first getting into comic books, Civil War was one of the first stories I read. I didn't know much about the Marvel Universe, but I'd heard of this event. However, having recently checked out its most overlooked tie-in, I've learned I never knew just how incredibly bloody it was, and most fans don't.
Civil War was a brutal hero vs hero conflict, as Iron Man and Captain America clashed over the idea of ing superhumans with the government. After a number of offensives and counter-attacks, Steve Rogers lays siege to Tony Stark's Prison 42, hoping to free all his captured allies. The battle spills out into New York, where Captain America finally surrenders, unwilling to put innocent lives at risk by continuing the fight.
However, while reading the tie-in epilogue comic Civil War: Battle Damage Report, I just learned he didn't surrender fast enough. The issue includes a memo written by Tony Stark which specifies that "47 non-powered humans were killed during the massive superhero battle that erupted in Times Square." These deaths aren't shown in the event itself, and they change the entire tone of the story.

Civil War's Alternate Ending Unleashed Iron Man's Ultimate Weapon
Marvel Comics' Civil War was a pivotal moment for Iron Man, but fans have now been given a glimpse into how wrong it really could have gone.
Civil War's Final Battle Had Major Civilian Casualties
Civil War: Battle Damage Report by Anthony Flamini, Ronald Byrd and Ronald Byrd
The final battle of Civil War is a massive fight between Tony Stark's pro-registration heroes and Captain America's resistance. Essentially every major Marvel hero is involved, and the fight ends with Captain America preparing for what appears to be a killing blow against a downed Tony Stark. However, Cap is tackled by emergency workers who desperately want to end the battle. Realizing he's putting everyday heroes at risk, Captain America surrenders, allowing himself to be arrested without backing down on his ideological stance.
I always thought Captain America's surrender was a little overblown, as he cries over what the battle has done to New York. The main event series' central death is William Foster's Goliath, though tie-in stories also killed D-list figures including heroes Typeface and Bantam, as well as villains Slyde, Jack O'Lantern, Goldbug and Plunderer who are gunned down by the Punisher while pursuing an injured Spider-Man. While Goliath's death is tragic, it's also presented as the one major fatality of the conflict.
However, knowing that 47 people were already dead in the battle Captain America created, his surrender is way more impactful. In fact, it enhances the story, depicting the superhero Civil War as a conflict that is growing more and more out of control and heading for disaster.
Captain America Surrendered, But Only After Nearly Fifty People Died
Civil War #7 by Mark Millar and Steve McNiven
Early in Civil War, Captain America warns that if Iron Man continues to push him, he'll learn the difference between traditional superhero fights and warfare. Similarly, tie-in series Civil War: Front Line contrasts events in the superhero war with real-life conflicts, including war poetry and depictions of deaths in historical wars. For today's reader, it's overblown and borderline offensive to compare a superhero event to real-life deaths in combat. However, the deaths of 47 people transform the story, making it so that Civil War is about the superhero conflict becoming a literal war.
The 47-person death toll changes how I understand Civil War, because it turns Goliath's tragic death into far more of a warning of what was going to come from hero fighting hero. If Iron Man or Captain America had been willing to step back from violent conflict, almost 50 people would still be alive. That's different to the massive property damage that the Civil War series actually depicts - a disquieting visual, but something that's far more par for the course in superhero stories.
Interestingly, 2015's Civil War volume 2 shows what would have happened if Cap hadn't stood down. This alternate reality depicts an America split in two by Captain America and Iron Man's war, with many heroes and villains dying as the two rival groups drag their war on for six years. This shows what Civil War's New York battle could have started, rather than what it ended, with those 47 deaths showing what happens when superhuman conflict is allowed to run out of control.

Marvel Comics: 10 Harsh Realities Of Rereading Civil War
Civil War was one of the biggest Marvel events in the 2000s, but readers will be faced with some harsh realities when rereading the iconic series.
Civil War's Huge Death Toll Exists in Continuity Limbo
Ultimately, Readers Get to Choose Whether the 47 Deaths Happened
Civil War: Battle Damage Report's detail of 47 deaths changes Civil War in multiple ways. On one hand, it gives the conflict greater significance, creating a very real cost to Iron Man and Captain America's conflict beyond the tragic death of a single hero. On the other, it further plays into the event's self-serious tone, making it difficult to redeem Marvel's heroes when their internecine squabble cost so many lives. Worse still, Captain America later recalled the Superhuman Registration Act when he was asked to take charge of SHIELD, meaning all those people ultimately died for nothing.
But while Civil War: Battle Damage Report's reveal changes how I understand the event, I find it hard to truly consider this detail canon. Later stories haven't treated Iron Man and Captain America's conflict as if it killed 47 people, with most of the focus placed on Goliath's death at the hands of the Thor clone known as Ragnarok. If you didn't read one obscure event tie-in (which is mostly an encyclopedia of the characters involved, presented as briefing materials after the 'war'), you'd never know how devastating Civil War's final battle was meant to be. In some ways, this is a good thing - fans get to choose whether they 'believe' this number or not depending on whether it enhances the story for them.
Marvel lore is dense, and there are always surprises lurking in back issues. I was genuinely surprised that, at least at one point, Marvel intended Civil War's final battle to have claimed almost 50 lives, turning Captain America and Iron Man's ideological battle into one of the bloodiest moments in Marvel continuity.