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Co-op is also available in the Halo Wars campaign, both via system link and over Xbox LIVE. However, unlike in other Halo games where co-op is available, it is not possible to play local co-op in Halo Wars.
Co-op gameplay basically involves two or more people playing through the campaign as a team. If one player dies, they can come back to life, or "respawn". The player who dies has to wait until the player(s) who is/are still alive is/are not engaged in combat and is/are at a safe location before they are able to respawn. If all players are killed, they will restart at the last saved checkpoint. Additionally, in Halo 2, if two players are playing on Legendary difficulty and either of them die, then both of them have to start at the last checkpoint. In any of the other Halo games, nearly all players will attempt Legendary on co-op, because Legendary is often too difficult to attempt alone. When a player respawns in Halo: Combat Evolved, they are always equipped with an assault rifle and a plasma pistol. In the sequels, each level has a designated pair of weapons that respawning characters start with.
In Halo: Combat Evolved and Halo 2 co-op, when playing a level involving the Master Chief, both players will control two identical copies of the Chief; when playing a Halo 2 level involving the Arbiter, both players will control identical Arbiters. Halo 2 was planned for online co-op mode for upto two players on xbox live, but due to time constraints Bungie was not able to perfect online co-op for Halo 2, the user interface still contains clues as to how Halo 2 would have had co-operative game mode, and it's even more apparent modders were able to activate this game mode.
In Halo 3, when there are two players, player one plays as the Master Chief and player two controls the Arbiter. Further, in three or four player co-op, player three controls N’tho 'Sraom and player four becomes Usze 'Taham. Two player co-op can be played on split screen mode in all three Halo games, but three or four player co-op in Halo 3 is only available over system link or Xbox Live, with at least two Xboxes with two players on each.
Co-op is also available in Halo 3: ODST; in the Mombasa Streets sections of the campaign, when player one controls the Rookie, player two controls an identical generic ODST. During flashback missions, Player one controls the main character of the flashback, but player two still controls a generic ODST, even if the character is accompanied by other members of the squad. For example, in NMPD HQ, the flashback is from the perspective of Romeo, who is constantly accompanied by Buck. In co-op mode, player one controls Romeo, but player two controls a nameless ODST, leaving Buck as an NPC. Just as in Halo 3, two players can play on one Xbox with split-screen, but up to four players can play at once via Xbox Live or system link.
In gameplay terms, each player can access the map and drop their own waypoint, which will show up on everyone's map, but only on the compass of the player who placed it. In addition, all players download all Audio Logs, no matter who actually accesses the terminals. Only one log can play at a time, and any player can stop the playback or start the playback of a different log.
Finally, ODST adds a new co-op multiplayer gametype called Firefight, in which players are allowed to pick their character, choose their personal emblem, and decide whether or not their character is wearing their helmet (with the exceptions of the Rookie, who must wear his helmet, and of Johnson, who cannot wear one). For more details on Firefight, see its main article.
Co-op gameplay is very different from single-player gameplay; there are several new strategies and alterations present. Starting and respawn weapons tend to vary between single-player and co-op, as do the starting weapons of each character and the strategies that can be used in co-op -- some of which are documented below.
The "coward method" is basically having one player hide in a safe spot while the other player charges forward and kills everything with impunity, charging again and again as they die until no opposition remains. This method of progressing is generally frowned upon in the community, but it is still commonly used to beat the campaigns on Legendary. It cannot be used in Halo 2 on Legendary, however, as if one player dies, both must restart from a checkpoint.
This method can also be used by one person playing as two characters on co-op, provided that they can place the "coward" in a safe, unreachable place.
This method involves all players charging at the enemy. Though this method is harder than others, it will make for a more enjoyable playing experience, and will be a true test of skill to get through levels. To use this technique successfully, players will need to coordinate movements with each other to avoid wounding or killing each other with splash damage or friendly fire.
This Halo 3-specific method consists of attempting to get through the level as quickly as possible, while also trying to get the most points possible. It often involves using vehicles and ignoring many enemies that you may otherwise be inclined to shoot. The reason players do this is generally because they want the Legendary symbol for their service record; they may also wish to get the meta-game achievements by relying mainly on the time bonus for the level.
This method makes up for the 2 or 3 weapon limit and has players focus on specific aspects of combat, such as enemy types or positioning. One of the most ideal strategies utilizes one of the players using weapons less resistant to shielding while another player uses weapons less resilient to non-shielded enemies. This way, shielded enemies would lose their shielding rather quickly and become vulnerable to the other player who would have the proper weapon equipped to finish off the enemy, effectively removing the need to constantly switch weapons. Ideally, the player assigned to shielded enemies would utilize Plasma Pistols for their overcharged shot while the player assigned to take out the non-shielded enemy would utilize headshot weapons, namely a Battle Rifle or Covenant Carbine, forming a two person Noob Combo.