Treyarch will be fixing the rigidly linear 4-6 hour singleplayer campaigns that the previous Call of Duty games have had. The missions in the game allow the player to choose different paths that change the course of the mission, but it doesn't stop there, BO2 will also be featuring an open-ended "Strike Force" game mode in the campaign.
During the campaign the player is given the option to take on various special ops missions around the world. After choosing a mission, it will act as a branch for the story and the structure of the campaign. The player's squad can die during the mission, but it's not game over - the death will be included as a part of the story.
The player will not be taking on the role of a specific character during the Strike Force missions; instead the player controls the entire squad. The missions themselves are open ended, meaning the player can choose to complete it anyway he or she wants. The player can order the whole squad or take control of individual squad members, the player can also control the flying Quad drones and CLAW tanks.
Strike Force isn't a separate mode and won't be featured in multiplayer at this point, but Treyarch is not taking the option off the table.