From Halopedia, the Halo wiki
Arcadia Outskirts is the fifth campaign level of Halo Wars.
After evacuating numerous civilians inside Arcadia City, Sergeant John Forge, his Marines, a Scorpion tank and Red Team retreat to a nearby impact crater, the closest defensible position. However, along the way they encounter large numbers of Covenant near their route and pursuing them. The UNSC forces manage to reach the crater and clear it of Covenant forces.
The UNSC forces construct a Firebase in the crater and begin building up their forces as they defend the crater, joined by more retreating forces brought in by Pelican dropship. The UNSC forces are forced to hold for around twenty minutes against massive waves of Covenant troops and vehicles, with the Covenant eventually constructing a Mega Turret nearby. Finally, Forge and his forces manage to hold out long enough for SPARTAN Team Omega to arrive to reinforce them.
With the help of Team Omega, the UNSC forces launch an attack on a nearby Covenant citadel, the main Covenant base in the area. After the base is destroyed, Team Omega departs the area and Forge is recalled to the UNSC Spirit of Fire because they've got a new problem.
Forge and his troops are evacuating from the city, under Covenant pursuit.
UNSC forces reach the crater only to find Covenant forces occupying it.
UNSC forces clear out the crater.
A D20 Heron deploys a Firebase in the crater.
For the rest of the level, Pelicans fly from the north side of the map to the crater and deploys reinforcements.
If the Mega Turret is destroyed
If a second base has been established:
At the 20 minute mark of the level:
Two Pelicans arrive at the crater and drop off six Spartans.
The UNSC forces launch an attack on the Covenant base north of the map.
The Covenant base is destroyed.