A new Invaders comic book series pits Captain America against his former ally, Namor the Sub-Mariner. The series will include both modern day and World War II storylines. In the Marvel Universe, "The Invaders" generally refers to a team of Golden Age superheroes who worked together to fight the Third Reich in World War II.
The founding members are Captain America, Namor, the original Human Torch, Toro, and Bucky (now known as the Winter Soldier). Though a Namor the Sub-Mariner movie may not be on the table anytime soon, the Avenging Son's presence in the comics is only getting stronger. Namor is a member of Jean Grey's new team of mutants in X-Men: Red, and will reunite with Doctor Strange, the Hulk, and Silver Surfer for an ongoing Defenders series in December. The character is also the antagonist in the latest arc of The Avengers, where Namor is seen sporting a whole new look.
Captain America and Namor will face off in Invaders, a new celebrating Marvel's 80th anniversary which will release "throughout" 2019, suggesting it will be an ongoing title and not just a limited series. Captain America will be joined by Winter Soldier and Human Torch as he hunts down Namor, who will attempt to "unleash a new deadly threat". The series will tell two different stories, with one focusing on the Invaders' battle with Namor, and the other on their team-ups in World War II.
As two of Marvel's oldest characters, Captain America and Namor have a rich history together in the comic books. They knew each other as allies in World War II, but their relationship changed after Captain America was revived in the Silver Age. Namor's repeated attempts to wage war on the surface worlds have often caused the two to come to blows. However, Captain America is among the few humans Namor actually respects, which is one reason why the two continue to work together from time to time. Against the wishes of the government—and to the shock of the American people—Captain America once even arranged for Namor to join the Avengers, which is a strong sign of how much they respect each other, despite their many differences.
Based on the description for the series, it appears Cap and Namor aren't on such good terms these days. Their upcoming clash likely stems from the storyline brewing in The Avengers, which sees Namor lead the Atlanteans into battle against the humans after some of his people are murdered by Roxxon Energy Corporation. It's possible that Captain America will be able to appeal to Namor's good nature and convince him to give up on his crusade against humanity.
Invaders #1 launches in January of 2019.