Call of Duty: Black Ops 6 will require a constant internet connection, even when playing on your lonesome in the single-player campaign.
An Activision blog dedicated to the game's different editions reveals that the always-online requirement comes from "texture streaming" technology that's used throughout every game mode, including the campaign. Activision explains the texture streaming tech works to "deliver the highest quality visuals while also reducing the game's overall storage space on your hard drive."
