Like its chief competitor, the Battlefield series, the Call of Duty series of games is driven by simulated military conflict between the United States and other world powers. The difference between these games extends beyond their respective engines (e.g.: Frostbyte was used for Battlefield 3 and 4 as opposed to modern variants of the Quake engine having been used for Call of Duty). While Battlefield hones in on realism, Call of Duty is more of a traditional first person shooter, in that it focuses on fast-paced gameplay where dying is nearly completely inconsequential. In Battlefield, the player respawns after being fragged, but also respawns in some random location on a very large map. The inherent penalty for losing in Battlefield is to be displaced from your last position, where you probably were fighting for a control point. In Call of Duty, the gameplay is focused on killing, respawning, and killing again until other team runs out of tickets, which are total respawns for the overall team.
Beginning with the Black Ops series, Call of Duty began to take a turn toward more fictional, sometimes even sci-fi story arcs. It drew good contrast from its Battlefield competitor; which, though equally fictional, focuses more on real world settings. Both games are speculated to have been a source for military recruitment. One may be hard pressed to find a person serving in the military these days who has not been inspired by these games in some capacity or another.
by James Hill