Warning: Contains spoilers for Avengers #54
One of the oldest friendships in the Marvel Universe is officially ending, as tensions in the Avengers drive a wedge between Captain America and Namor the Sub-Mariner, who have been fighting together since the Second World War.
Namor and Steve Rogers met for the first time during the war. After Nazi divers were found near Atlantis, Prince Namor was sent to the surface to help the Allies in their war efforts. He joined the Invaders, a team of super-powered individuals composed by himself, the original Human Torch, his sidekick Toro, Captain America, and Bucky. Together they fought against the Axis and their superpowered agents and, in the midst of the greatest war the world had ever seen, Namor and Steve Rogers forged a bond that would last for many decades. Namor would actually play a pivotal role in Captain America's revival in the modern age. In his wanderings, Namor found a tribe who was worshipping a mysterious figure encased in ice. Enraged by their idolatry, Namor threw the ice block into the ocean, where the Avengers found it and discovered it contained Steve Roger's body in suspended animation.
Unfortunately, Namor's dual nature as an Atlantean and a Human led to a difficult relationship with the surface world. Sometimes he was an enemy, but more often he was a hero. Namor even joined the Avengers for a time, and Captain America's leadership and the personal respect and trust they have for each other were the only things that could keep the Avenging Son's difficult personality in check. Namor fought on Cap's side during the first Civil War, but after Steve's death, Namor walked a darker path. He joined Norman Osborn's cabal, even if he betrayed them, and later fought against Cap and the Avengers after being possessed by the Phoenix, during the Avengers vs X-Men event. It was then that Namor committed an act that Steve could never forgive, killing an untold number of innocent people in Wakanda. Despite everything they went through, however, Namor still holds the utmost respect for Steve, but judging from Avengers #54 (by Jason Aaron, Juan Frigeri, and David Curiel), it appears that the feeling is no longer mutual.
Namor recently clashed with the Avengers, when he declared that the seas were off-limits for humans, antagonized the surface world, and put Atlantis under martial law. When the Avengers confronted him, he single-handedly held off the likes of Thor, Captain Marvel, the Ghost Rider, Iron Man, and She-Hulk, and only stopped when Captain America asked him to. However, after She-Hulk saved Atlantis from the explosion of a Gamma Bomb, Namor promised he would do anything to repay her, and Jen asked him to join the Avengers. Steve, however, is fed up with Namor's cocky attitude and wants him to be accountable for his actions. He tells him that he is officially under Avengers' custody, and when Namor calls him "Steve" he responds that it's "Captain Rogers" for him.
While Captain America seems a little cold-hearted in this exchange, he does have a point. Since the Wakanda massacre, Namor has done a lot of questionable things, including allying himself with Thanos. Only because She-Hulk "guilted" him into joining the Avengers, it doesn't mean that his crimes are forgotten. Hopefully, Namor will be able to earn Captain America's trust and respect back as an Avenger, but it's still sad to see Marvel break up its oldest friendship like this.