When Captain America and Iron Man fought on opposite sides of Marvel's Civil War event, they tore the comics universe apart in such a way that the repercussions are still felt even after fifteen years. The fighting was long and brutal (and even ultimately ended with Captain America's untimely death, though he eventually recovered from death as superheroes are wont to do). But in Dark Reign: Fantastic Four #2, the Fantastic Four discover that Steve Rogers and Tony Stark could have patched things up in a slightly more romantic fashion.
The Civil War event began with an otherwise-unremarkable fight between the New Warriors and a group of villains that ended in disaster when the supervillain Nitro exploded, destroying a school and killing over 600 people. The incident spurs intense debate about masked superheroes in America operating without any oversight or training, and the Superhuman Registration Act is proposed: a law that would require all superheroes to reveal their identities to the government. Additionally, mandatory training would be required for some, and any superhero or superpowered individual who did not comply would be arrested (and in some instances, sent to the Phantom Zone).
Iron Man strongly supported the SRA while Captain America was staunchly against it; he went underground along with his superhero supporters and the Civil War proper began. The event culminated in a massive battle in which Captain America realized the prolonged fight between him and Iron Man served no purpose, and willingly turned himself in (he would later be assassinated by a sniper after the event). But Reed Richards, looking for other outcomes via the Bridge - a machine that allows him to view alternate realities - saw a universe in which Captain America and Iron Man settled their differences not with hate, but with love. Peace was achieved in part due to "...the romantic involvement of Captain America, Steve Rogers, and the Iron Woman, Natasha Stark."
Note that the Armored Avenger of Earth-3490 is named Natasha Stark; this implies that there is no Black Widow in this universe and Natasha Romanoff assumed Tony Stark's role as a brilliant inventor and weapons manufacturer. Unfortunately, Marvel's decision to make this Iron Man a woman means the Steve Rogers and Tony Stark with which fans are familiar are not technically lovers in this universe. Perhaps Marvel wanted to avoid writing these characters as gay, which is unfortunate; this is an alternate universe after all, and the heterosexuality of Iron Man and Captain America were never core characteristics of these heroes to begin with.
Reed Richards continued to move forward with the Superhuman Registration Act without further bloodshed on Earth-3490. Other universes saw the death of one or both of the heroes - but the prime-616 universe saw the death of others still. Regarding Captain America and Iron Man's violent actions in Civil War in Earth-616, perhaps Gottfried Leibniz was wrong: this is not the best of all possible worlds.