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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: 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, Halo: Reach, and Halo 4, the number of players permitted has changed to allow up to four players and the use of Xbox Live.
In Campaign Mode, Terminals can be found hidden away. They can provide secret information about events, filling the player in on Halo's backstory, such as Halo 3's Terminals giving information of conversations between the Librarian and Ur-Didact/Iso-Didact. They are found in Halo 3, Halo: Combat Evolved Anniversary, and Halo 4. The equivalent of terminals also can be found as Data Pads in Halo: Reach and as Audio Logs in Halo 3: ODST, giving information about the story of Sadie Endesha as the Covenant attack the city of New Mombasa. Halo: Combat Evolved Anniversary's terminals are in video form, and track 343 Guilty Spark before, during, and after the John-117 lands on Installation 04, and the events of Halo: Combat Evolved. Unlocking all of these Terminals allows them to be re-viewed on Halo Waypoint, instead of forcing the player to revisit the areas. At the end of each of Halo: Combat Evolved Anniversary's Terminals is a code, written in Forerunner Symbols. Entering each of these codes into a secret page on Halo Waypoint rewarded the player with Credits in Halo: Reach, with 100,000 Credits being rewarded to the played upon entry of the final code. Doing so also unlocks Terminal 11: Threshold for view. In Halo 4, when a player finds a terminal, it can only be viewed in Halo Waypoint.
- Halo: Combat Evolved starts with John-117 leaving a cryo pod and Halo 3 ends with him entering one.
- Halo 3 is the only game where campaign co-op mode can be considered canon, since its previous games features 2 John-117s or Arbiters, and there are only 6 members in Alpha-Nine and Noble Team. Halo 3's second player is the Arbiter, third as the Sangheili N'tho 'Sraom and fourth as Usze 'Taham.
- Most games have a unique, unofficial, difficulty known as the Mythic difficulty, which is Legendary with all Skulls activated.
- This difficulty is officially known as LASO (Legendary All Skulls On) and has been named such in achievements and challenges.
- 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 Halo: Reach starts and ends with the player in space. In Halo: Combat Evolved, the game starts with John-117 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 in 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 the late Sergeant Forge. Halo 4 starts with John-117 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 Halo: 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.
- In Halo 2's campaign, the player plays as John-117 for 8 (7 if one does not count The Armory) levels, while the player plays as Thel 'Vadamee for 7 (6 if one does not count The Heretic) levels.
- Halo: Spartan Assault has the longest campaign in the Halo series so far (its campaign features five "operations", each with five levels, adding up to a total of 25 levels).
- Both Halo 2 and Halo Wars have the second-longest campaigns in the Halo series 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 second-longest campaign in the series so far, and Halo 2 has the third-longest campaign in the series so far.
- If one does not count the Halo 4 levels Prologue and Epilogue (both of which are cinematics), then Halo 4 has the shortest campaign in the series so far, with 8 playable levels.