When Hank Pym over his abusive treatment of his wife Janet van Dyne.
In the Ultimate Universe - a parallel Marvel dimension outside of mainstream Marvel continuity that allowed fans to follow modernized versions of their favorite characters - Captain America and Hank Pym were both of the Ultimates (the Avengers in all but name.) Following their first major victory against the Hulk, Hank Pym and his wife Janet van Dyne, also known as Giant-Man and the Wasp, got into an argument which escalated after Hank struck Janet. When Jan fought back as the Wasp, Hank not only sprayed her with bug spray but also used his Ant-Man helmet to command a legion of ants to attack her while she hid in her miniature form. Janet was hospitalized, while Hank fled the scene, making himself a fugitive. The old-fashioned Captain America couldn't let this stand and secretly used SHIELD resources to find Hank with the purpose of bringing him in, with or without Fury's approval.
In Ultimates #8-9 by Mark Millar and Bryan Hitch, Steve Rogers tracks Hank to a bar where the size-changing "hero" offers a few excuses, which are met with Steve's insults and fists, hoping to force Hank to grow bigger for a fair fight. Steve gets his wish as a naked, giant-sized Hank Pym tosses him through a nearby building, ready to put down the old-fashioned Boy Scout. Despite Hank's advantage in size and strength, Steve's training and powers thanks to the Super Soldier serum help him take on Giant Man, their fight spilling over to a nearby construction site.
Writer Mark Millar sought to re-interpret the most infamous moment in Hank Pym's superhero career, turning the tangled continuity of Hank's original slap into vicious, potentially deadly abuse. When Hank attacks Janet, it's with the chilling line, "You shouldn't have made me look small," so it's no coincidence that Captain America forces him to fight at his biggest before literally putting Hank into traction. When they fight in the construction site, Hank seems to blame Steve for the entire situation, accusing him of flirting with Janet and now trying to tell him how to live his life. Millar depicts Hank not as someone in the throes of temper, but rather seized by the cold rage of entitlement and narcissism, only able to see the situation in of what it means for him.
Captain America uses the fight to demonstrate a lesson in the difference between overwhelming power and actual skill, using Giant-Man's size to his advantage and eventually dropping construction supplies on him, badly injuring Hank just as SHIELD arrives to order Cap not to engage. Although Steve would go on to date Jan, both she and Fury ultimately criticized Cap for his macho response, pointing out that his brutal attack on Hank was never truly a fair fight, and actually helped motivate Hank to betray the team to the Liberators in The Ultimates 2, perpetuating violence on a larger scale.
In its brutality, this fight helped mark the Ultimate Universe as a tonally different place to mainstream Marvel, and also imagines Captain America as not just an enduring icon, but someone whose sensibilities are literally from a different age. The scene also helped to cement the perception of Hank Pym as an ab - something which likely motivated his depiction as a retired former hero, rather than the star, of 2015's Ant-Man movie. As comic book fights go, not only was it particularly violent and mean-spirited, but it cast a long shadow over both Captain America and Ant-Man going forwards.
