Thanos invades Atlantis instead of Asgard in an effort to find the Space Stone
Back then, Howard Stark had never found that infinite stone when he searched for Steve Rogers. Atlanna is the person who discovered it kept it for decades.
Her two sons worked together with joint forces from Altantis, Xebel, Fishermen, and Brine.
Karathen added as a backup.
Thanos has the Black Order by his side.