In recent years, several of the Marvel Universe's most important nations have taken center stage in a bid to control the future of the planet. Unlike these other nations that only formed relatively recently, with one engaging in their first actual war, there are two nations that have stood above the rest for centuries: Wakanda and Atlantis.
These two nations have served as the technological height of the planet, separating themselves from the world and choosing to only get involved in major moments of crisis. With these similarities, one would assume that the Black Panther and Namor, the leaders of these respective nations, would be able to share in their similarities. However, the contentious relationship between the two and their nations nearly led to the end of everything.
This conflict stemmed from the two kings' own allegiances to their respective teams: the Avengers and the X-Men. In Avengers Vs. X-Men, the two teams found themselves on different sides over the pending arrival of the Phoenix Force. Black Panther only saw a potential threat, while Namor saw hope for mutants. The battle intensified when members of the X-Men were imbued with the Phoenix Force, leading the Avengers to go on the run and hide in Wakanda. After a fierce battle, Namor used his power to unleash a tidal wave on T'Challa's home. This crossed the line, leading to Namor's expulsion from both the Phoenix Five and the X-Men.
After this attack, Wakanda rebuilt, while it slowly began its preparations for war against Namor. Yet, capturing a villain known as Black Swan, T'Challa learned of the multiverse's collapse in events called Incursions and made the choice to reconvene the Illuminati, a group that he had once rejected, and meant turning to one of its original members, Namor. The Illuminati regularly met in Wakanda's undead city, Necropolis, to devise solutions to stave off destruction, and T'Challa and Namor found themselves on the same side.
Aware of the larger and coming threat, Namor decided to sue for peace, hoping that T'Challa could convince his nation to agree to terms that would put an end to the potential conflict. T'Challa agreed to lend his support but failed to convince Shuri, queen of Wakanda, deciding that Wakanda must show its strength and what happens to those that attack them. From there, T'Challa called Namor under the guise of peace talks, only to tell him that his hope for peace had failed and that the Wakandan army had destroyed the city of Atlantis. Returning to his country, Namor mourned for his people until a member of Thanos' Black Order, Proxima Midnight, arrived before him.
With his defenses already crumbled, Namor was forced to bend the knee to Thanos, as it was the only way to defend what was left of his people. During his interrogation by Proxima, Namor saw the opportunity for revenge and lied to Thanos by saying the Infinity Gems were in Wakanda. Given Thanos' obsession with the Gems, his forces were sent to destroy Wakanda. This lie resulted in the death of even more Wakandans and also led to the near destruction of the entire Earth, as the forces of Thanos uncovered the stockpile of doomsday devices created in perpetration for the coming Incursions. While the Avengers battled Thanos and his army, the Illuminati fought Supergiant and clean up the mess created by Namor. The team only succeeded thanks to the Inhuman, Lockjaw. This would have consequences between Namor and Black Panther leading to the splitting of the Illuminati and the rise of the Cabal. It would take the literal end of the universe, in the midst of Secret Wars, until the pair could fully work together.
Now, it seems they continue to be on another collision course. As Black Panther continues to lead the Avengers, Namor's war with the surface world has reached a turning point with the arrival of the Phoenix Force, which he called upon to aid him in his current vendetta. Since the Phoenix Force was partly responsible for Wakanda's initial destruction, Namor could hold plans to now turn its power towards the rest of the world.