The Call of Duty series has existed since 2003 and in addition to various spin-offs, including for the handheld systems Nintendo DS and Sony PSP, 14 major titles of the series have now been released. Ever since the first part of the series, great emphasis has been placed on the online multiplayer mode in addition to the single-player campaign, which is also reflected in the development of the shooter series. Since the first Call of Duty, World at War, which was developed exclusively by Treyarch, each part also included cooperative challenges that you could play together with friends. The latest installment, Call of Duty: World War II, also features an extended multiplayer mode that puts you and your friends in the combat zones and famous battles of World War II.