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Campaign mode is the single player gaming aspect of Halo: Combat Evolved, Halo 2, Halo 3, Halo 3: ODST, Halo: Reach, Halo: Combat Evolved Anniversary, and Halo 4, where the player takes control of one or more playable characters at a time through the canonical story of the Halo trilogy timeline, and the single player aspect of Halo Wars, where the player takes control of a UNSC Marine Corps force.
In Halo: Combat Evolved, Halo 3, and Halo 4, the player plays as the Master Chief Petty Officer John-117, a SPARTAN-II. In Halo 2, the player plays as both John-117 and the Arbiter Thel 'Vadam(ee), a Covenant Sangheili, which allows the player a more personal view of Covenant society. In Halo 3: ODST, the player plays as a variety of Orbital Drop Shock Troopers (ODSTs). In Halo: Reach, the player takes the role of Noble Six, a SPARTAN-III whose name and past are unknown.
In Halo Wars, the campaign has a different gaming aspect. The game is a strategy video game, and the player takes control of a base, multiple troops and several vehicles. It is up to the player to decide where a squad will go or attack, when a building will be constructed, etc. Again, the game follows the canonical storyline of the Halo Universe.
The Campaign Mode can be played on four difficulty levels, listed in order from most difficult to the easiest: Legendary, Heroic, Normal, and Easy. In addition, Halo: Combat Evolved and Halo 2 have a co-op Campaign option permitting two players to play through campaign either in split-screen or via system link between two Xbox gaming consoles. For Halo 3, Reach, and Halo 4, the number of players permitted has changed to allow up to four players and the use of Xbox LIVE.
- Halo: Combat Evolved starts with Master Chief leaving a cryo pod and Halo 3 ends with him entering one.
- Halo 3 is the only game where campaign co-op mode could be canon, since its previous games features 2 Master Chiefs or Arbiters, and there are only 6 members in Dare's ODST Squad and Noble Team. Halo 3's second player is the Arbiter, third as the Sangheili N’tho ‘Sraom and fourth as Usze 'Taham.
- Halo 2, Halo 3, Halo Wars, Halo 3: ODST, Halo: Reach, Halo: Combat Evolved Anniversary and Halo 4 have a unique, unofficial difficulty known to fans as Mythic, which is Legendary with all Skulls activated.
- In Halo: Combat Evolved, Halo 3 and Halo Wars, the endings of the three games end with a huge Forerunner structure getting destroyed: Halo: Combat Evolved ends with Halo Installation 04 being destroyed, Halo 3 ends with Halo Installation 04B firing (and getting destroyed by the effects, as well as possibly the Ark), and Halo Wars ends with a Shield World imploding.
- Every Halo game apart from Reach starts and ends with the player in space. In Halo: Combat Evolved, the game starts with Master Chief on the UNSC Pillar of Autumn and ends with him in a Longsword. Halo 2 starts with him on Cairo Station and ends with him on the Forerunner Dreadnought. Halo 3 starts off with him crashing towards Earth and ends with him stranded inside the UNSC Forward Unto Dawn. Halo 3: ODST starts with the Rookie on the UNSC Say My Name and ends with him on an ONI orbital facility. Halo Wars starts with Captain Cutter on the UNSC Spirit of Fire talking about the war on Harvest and ends with the closing of the cryotube of Sergeant Forge. Halo 4 starts with the Master Chief aboard the stranded Forward Unto Dawn and ends with him aboard the UNSC Infinity.
- Halo: Reach is the first Halo game to feature Matchmaking support in Campaign where instead of inviting or joining other players, it features the ability to search for other players. Campaign Matchmaking was made available to the public as of October 15, 2010.
- Halo 3 and Combat Evolved both end the same way with you blowing up Installation 04, a Warthog run, and then end up drifting in Space.
- When playing campaign of FPS Halo games on high difficulty levels, speed running can be used to complete levels quickly.
- Both Halo 2 and Halo Wars have the longest campaigns out of all the Halo titles so far (both have 15 levels total). However, if one does not count the Halo 2 levels The Heretic and The Armory (which consists of an opening cinematic and a tutorial, respectively), then Halo Wars has the longest campaign in the series so far.
- If one does not count the Halo 4 levels Prologue and Epilogue (both of which are cinematics), Halo 4 has the shortest campaign in the series so far, with 8 playable levels.