Back in 2010, X-Men and Avengers were beefing because a new red head girl with possible phoenix powers emerged and there was fear of another Jean Grey situation; so both sides wanted custody of her. During the beef, cosmic fukkery happened and 5 of the x-men, including Cyclops and Namor (technically a mutant), inherited cosmic phoenix powers. Avengers proceeded to get they ass tore up and had to take sanctuary in Wakanda, granted by T'Challa.
Cyclops told X-Men gang to leave Wakanda alone for the time being but Emma Frost, a telepath who was Cyclops girlfriend but was also fukking Namor on the side, pulled Namor aside and told him fukk them nyggas
, drown them
.
So in what became an infamous moment in comics, a Phoenix/cosmic powered Namor went rogue and sent a huge tidal wave to flood a large segment of Wakanda... killing thousands if not millions of Wakandans.
T'Challa, as you could imagine, was furious. He immediately divorced Storm (another infamous moment), who was riding with x-men gang but obviously didn't know what Namor/Emma planned, and declared war on Atlantis.
That's the cliff notes version