Halo: Spartan Assault co-op missions
From Halopedia, the Halo wiki
The Halo: Spartan Assault co-op missions form the co-op mode found in the Xbox 360 and Xbox One versions of Halo: Spartan Assault. With five missions, the mode focuses on preparing SPARTAN-IVs for the hypothetical return of the Flood. Unlike the campaign mode, which represents a recreation of canonical events, the Flood Missions are pure simulation. Unlike the series' other co-operative modes, Firefight and Spartan Ops, the Flood missions cannot be completed alone; players must play with a co-op partner over Xbox LIVE.
The mode's respawning mechanics work similarly to those in the campaign co-op of the other games in the series. If one player is killed, a countdown begins until they will respawn on their partner. Players can shorten this countdown by purchasing modifiers before the mission begins. Unlike other games, the living player does not have to be out of combat in order for respawning to occur. If both players are killed, the mission will end in failure, and the lead profile will have the option of restarting the mission or quitting.
Truth is... we let them do it.