Crossover events are one of the most basic phenomena in comics. Nowadays, it's virtually impossible to have a year go by without both of the Big Two having one.
They vary greatly. Some are considered legendary, hallmark comics. Others, absolute dreck.
Some crossovers are viewed as great milestones, so important they become bywords for world-changing events. In DC's case, the textbook example would be the Crisis on Infinite Earths.
For Marvel, I propose it's Civil War.
Civil War was huge. It was one of those very rare crossovers that lives up to the hype. I recall reading in a comic, months before, in some editorial, fourth-wall-breaking captions, that it was going to 'rip the fandom in two', and that is precisely what happened.
The concept - as always with great works - is simple. Many years earlier, the X-Men had had to deal with the 'Mutant Registration Act', legislation which would require of all people with mutant powers to register their identity with the government. Now, the 'Super-Human Registration Act', SHRA, was proposed, which would require all super-powered Americans to register their identity and powers, and to get a license to use their powers in public. Some people, especially young, inexperienced superhumans, would be required to attend a training camp to get the license.
This seems invasive and paternalistic, bordering on fascistic according to some, logical and responsible according to others. After all, people are required to have a license to drive a car or own a fire-arm and those can often do far less harm than certain super-powers. About half the super-heroes tended to agree, about half opposed, with the vast majority not 100% sure which side to pick... and therefore picking based on who was leading the opposing parties.
The heroes who opposed registration and ended up going 'underground', even posing violent resistance from hiding, were led by Captain America.
The heroes who supported registration, by Iron Man.
In the months that led up to the story, it was becoming clear that the government would sign the bill into law. Tony Stark lobbied for all that he was worth to stop this from happening, aided by his protegé Peter Parker, but then disaster struck: a group of young heroes, the New Warriors, who were filming their own reality TV-show, happened upon a group of supervillains who had been among the dozens who had escaped the super-prison the Raft months earlier. They engaged, but in the battle, one of the villains - the psychotic Nitro, who has the ability to explode and re-form himself at will - detonated with such tremendous force that over 600 people, including the New Warriors (with one survivor) and almost a hundred children were killed in the blast.
A horrified Congress passed the bill almost immediately... and Tony Stark now had to work to get the law accepted by the superhero community.
As said, this failed; about half of the heroes refused to accede, leading to basically a nationwide manhunt for Captain America and his allies.
In the event - comprised of a main series, several accompanying mini-series, one-shots and crossovers with regular series, making it possibly the most massive example of its kind in Marvel history - Tony was the face of the pro-registration side. And he suffered for it. Some fans agreed with him, many others denounced him as a fascist bully-boy fighting to oppress everybody.
While ostensibly neutral, in fact the comics fairly consistently depicted him as unreasonable, blinkered, and his methods brutal; superheroes who got arrested were sent to '42', a prison in the Negative Zone which was shown to be a deeply unpleasant place, akin to a Soviet gulag.
About halfway through the event, Spider-Man decided he could no longer side with the pro-Registrationers, and defected (this led straight into the events of 'One more day', probably one of the most despised Marvel comics of all time), leaving Stark even more alone and abandoned.
In the end, Pro-registration won, with Captain America being arrested and, just before his trial, assassinated by order of the Red Skull. The superhero community was and remained deeply divided for years, and the Marvel landscape was profoundly changed.
And at the heart of it all was Iron Man. This, I have to say, was what broke the hero. This was the cut-off point he never really recovered from.
The issues of his own comic dedicated to 'Civil War' were a two-issue story; in the first one, the villainous Spymaster broke into Stark Industries to assassinate Tony Stark, ensuring chaos and more profit for him, while also taking revenge on his hated enemy. It went differently; Happy Hogan managed to stop him, and while the villain got away to lick his wounds, Happy was left severely injured without anyone even knowing who had been the perpetrator. In the second issue, Happy was in hospital in a persistent vegetative state, life support systems keeping his organs going; there was no hope of recovery. Pepper told Tony that Happy, a former boxer (if you recall) once told her that that was no way to live; some old friends and idols of him, great fighters in their time, were little more than vegetables after they retired. Iron Man confronted Cap's side, including Spider-Man, and demanded to know if they were in any way responsible for what had happened; satisfied with their vehement denial, he let them go.
The issue ended dramatically: Happy died, it being heavily implied that Tony had used his Extremis abilities to shut down his life support functions, rather than letting him live on as a living dead man.
From here on out, things were never quite the same.