- “Captain America. Steve. I look at your handsome face...into your clear azure eyes...and, as ever, I feel the same guilty envy.”
- —Tony, Tales of Suspense Vol. 1 #1
While Stark was growing up, he was told of the legends of Captain America and was amazed when they found his body frozen in a block of ice. They first met when the Steve awoke from the ice and noticed his situation had drastically changed. He immediately asks Tony Stark—in the form of Iron Man—where he was.
Over the years, Steve and Tony form a deep bond with each other, becoming highly inseparable teammates and friends.
Earth-3490 is a universe that appeared in Dark Reign: Fantastic Four Vol 1 #2 as an alternative universe where the civil war ended differently. In this universe, the civil war was averted because Steve and Natasha Stark, Tony's 3490 counterpart, were romantically involved with each other and later got married. In this universe, Stony is canon.
Anthony Stark, once a brilliant and respected inventor living in the town of Timely, spiraled into alcoholism after the Union army used his repeating rifle to slaughter a large group of Confederates rather than let them surrender. He swore to never pick up a gun again. He broke his promise, however, after Sheriff Steven Rogers was shot and killed. He shot at Steven's killer in grief and rage, and later built a suit of armor to take down those involved in Steven's murder.
During Tony's childhood, his father, Howard, would constantly talk about the great Captain America, so as to inspire Tony to do great things in life. However, this greatly annoyed young Tony, who felt inferior to Captain America in his father's eyes.
When Nick Fury informed Tony of the Avengers Initiative, he learned that Steve was still alive. They met for the first time in Stuttgart, working together to subdue Loki. Tony commented on Steve's fighting skills and took a few subtle jabs at him.
Once the full team was assembled on S.H.I.E.L.D.'s Helicarrier and working to decode Loki's scepter, Steve and Tony's personalities clashed. They argued constantly, with Steve feeling that Tony was not taking their situation seriously, and Tony not taking Steve seriously as a leader. They verbally sparred with each other and nearly got into a physical fight, but were interrupted by an attack by Loki's brainwashed soldiers. Immediately forgetting their argument, the two worked together to fix one of the Helicarrier's rotors, which had been damaged during the attack. They later fought alongside each other during the Battle of New York.
Steve and Tony fought alongside each other to retrieve Loki's scepter from the HYDRA facility in Sokovia. After Ultron's reveal and subsequent attack, the two argued, disagreeing on the morality of creating Ultron without the rest of the team's knowledge.
After the disastrous battle at the salvage yard, Steve and Tony tried to relax by chopping wood outside Clint Barton's farmhouse. They discussed how Wanda Maximoff had easily pulled apart the team with her powers, but the discussion quickly turned into an argument as the topic of Ultron came up.
After the Battle of Seoul, Clint delivered the android body Ultron had been working on to Tony, hoping that he would destroy it. Instead, Tony enlisted Bruce to help his complete Ultron's vision. Steve then arrived to Avengers Tower with the Maximoff twins to stop Tony from doing so. The two argued and even briefly fought, but before it could escalate, Thor arrived and shot lightning into the android, bringing it to life.
Steve and Tony fought at each other's side during the Battle of Sokovia, resolving their previous differences to work together. Tony retired from active duty after the battle, and Steve told him that he would miss him.
Steve and Tony took opposing sides over the Sokovia Accords; Tony believed that the Avengers needed oversight, while Steve was wary of government agendas. They argued about the issue, though neither was willing to budge on their stance.
After Steve, Sam Wilson, and T'Challa were taken in for intervening in Bucky Barnes' arrest, Tony tried once again to persuade Steve into signing the Accords, promising that Bucky would receive mental health care instead of jailtime if he did. Steve nearly agreed, though he refused when he discovered that Tony was keeping Wanda on house arrest.
Steve and Tony faced off at the Leipzig-Halle airport. Despite the best efforts of Tony's team, Steve and Bucky managed to escape, flying off to Siberia to deal with the supposed Winter Soldier threat. Afterwards, Tony visited the Raft, where Steve's remaining teammates had been imprisoned after the fight. He approached Sam and asked him where the two were heading. Sam made him promise that he would go as a friend, and then told him.
Arriving in Siberia, Tony called a truce with Steve, and the three of them made their way through the facility as a team. The peace did not last, however; Helmut Zemo revealed himself and showed the three the security video of the Winter Soldier killing Tony's parents. Tony angrily confronted Steve, asking if he'd known. When Steve replied that he did, Tony backhanded him across the room and went after Bucky. Steve fought against Tony, trying to defend Bucky from him, their friendship deteriorating throughout the battle.
Tony nearly beat Steve to a standstill. When Steve refused to back down, Tony raised a repulsor for the finishing blow, but Bucky grabbed his leg, distracting him. This gave Steve an opening to beat Tony down and disable his armor. As Steve walked away with Bucky in tow, Tony desperately shouted that Steve didn't deserve his shield. Steve hesitated for a moment, but dropped the shield and continued away with Bucky.
Later, Tony received a package from Steve containing a letter and a burner phone with Steve's number in it. In the letter, Steve apologized for the events that had transpired, and assured Tony that if ever needed him, he was a phone call away. Upon receiving a call from Secretary Ross that Steve had broken into the Raft, Tony refused to answer, and allowed Steve to break out his friends.
Tony was reluctant to call Steve for help finding Vision. He pulled out the burner phone that Steve had sent him, but before he could call, Cull Obsidian and Ebony Maw arrived.
Although the two were shipped beforehand, the pairing didn't take off till after the events of the Civil War comic. Many shippers began to write fix-its or post story fan fictions that would have them get together. Shipping in the films took off based on their interactions being read as belligerent sexual tension, although some writers would use their knowledge of the comics to influence the way they wrote the characters in the MCU.
On AO3, it is the most written ship for Tony and the second most written for Steve Rogers. It is the most written ship in The Avengers (Marvel Movies), The Avengers All Media types and Iron Man (Movies) tags, and the second most written in the Marvel Cinematic Universe and Captain America (Movies) tags.