What was the first Call of Duty game to include the Zombies gamemode?

The first Call of Duty game to include the Zombies gamemode was Call of Duty: World at War. This game introduced the popular cooperative mode where players fought against hordes of undead Nazi zombies. It became a huge success, captivating players with its intense survival gameplay and eerie atmosphere. The Zombies mode spawned a series of sequels and is now considered an iconic part of the Call of Duty franchise, with its own dedicated fan base.