Alpha Base is the first campaign level in Halo Wars. During the final stages of the Harvest campaign in February 2531, the UNSC Marine Corps firebase, Alpha Base, has been overrun by Covenant forces. Sergeant John Forge has to pull together the remaining UNSC forces in the area and mount an attack to retake Alpha Base.
A voiceover by Captain James Cutter describes the five-year long campaign to retake the Outer Colony of Harvest from the Covenant. In the present, on February 4, 2531, the UNSC Spirit of Fire establishes orbit around Harvest where Sergeant John Forge reports that the Covenant have uncovered something in the ice where Professor Ellen Anders reports that scans of the planet's northern polar region show a structure beneath the ice. To Forge's annoyance, Anders insists on going to the ground to see what the Covenant are up to for herself while Forge is ordered to pull back to Alpha Base while Cutter sends him some backup.
Forge discovers that Alpha Base has been overrun and destroyed by Covenant forces and is ordered to rally nearby pinned-down Marine squads and retake the base. Rescuing the squads, Forge leads an attack on the ruins of Alpha Base, destroying a shield blocking entry and engaging the Covenant forces holding the position. Aided by a carpet bombing by Shortsword bombers that destroys the enemy vehicles, Forge and his men clear out the Covenant and retake Alpha Base. Forge is then ordered by Cutter to scout the area.
Harvest comes into view, its surface covered in peaceful colors of green and blue.
Several marines race through a battlefield within the remains of a destroyed UNSC city. A Marine dies when a nearby wall explodes from a passing Banshee's ordnance, and debris and plasma engulf him. The surviving Marines recover quickly and start to move forward.
They confront a Lance of Elites whom are separated by a wall of debris, a few of the marines yell out as they arrive. A Marine quickly joins in and starts to fire upon the Elites, only to be shot by a plasma bolt to the chest. The rest, despite being outnumbered manage to gun down several of the Elites, as another marine gets pierced in the shoulder with a needler round. Then a Warthog runs the Elites over. Cuts to the rear of the Warthog; it is driving through a battlefield with the gunner firing the turret.
Three marines fire their weapons, the last one firing a shotgun from the hip. Cuts to several Pelican dropships arriving, with several Warthogs racing through the drop zone. Marines then pile out of one of the landed Pelicans. Cuts to a Marine hitting a Sangheili from behind with his Assault Rifle. The Sangheili falls down and is shot dead by another marine. Shifts to a panorama, showing nine Marines cheering and a Warthog overlooking the battlefield; a destroyed city with a burning Covenant destroyer in the distant sky, presumably shot down by UNSC forces.
A Sangheili impales a Marine with its Energy Sword. UNSC troops are overwhelmed by plasma fired by Elites and Banshees. Shifts to a scene where three Sangheili jump over a crumbled wall, igniting their energy swords. A dead Marine lies nearby.
Two Covenant destroyers begin to glass Harvest. Damage from orbital bombardment is visible, represented by several rings of burnt land.
A wounded marine crawls through the shattered wasteland with a dark sky and debris. The Marine attempts to crawl to a rifle lying on its side, when a sword-wielding Sangheili approaches and impales him.
Directly overhead, a Covenant destroyer charges up its energy projector, and unleashes its deadly beam.
The screen fades to black.
The partially glassed surface of Harvest is shown, with the UNSC Spirit of Fire moving thorough debris in orbit.
FEBRUARY 4TH 2531
20 YEARS BEFORE THE HALO EVENT
Pelicans are dispatched from the UNSC Spirit of Fire and they head to the surface of Harvest. UNSC D20 Herons soon follow. The UNSC fleet is approaching a large field of shattered debris in orbit, presumably from wrecked UNSC vessels from previous space battles. The surface of the shattered planet, Harvest is then revealed. Damage from orbital bombardment is seen quite clearly, in the form of massive rings of scorched earth and ash choked seas on the surface. The view is soon headed into the bridge of the Spirit of Fire.
Anders activates a map of the northern polar region on the holo-table.
Anders ignores the Sergeant and leaves the bridge.
The Captain deactivates the map, returns to observing Harvest from orbit.
A Marine fires from a tower that is being overrun. Several plasma bolts hit under where the Marine is shooting from impact, followed by an explosion behind him, killing him. UNSC forces in Alpha Base are retreating through the gate, and those who stayed are quickly killed by the Covenant.
Multiple units of Marines flee from Alpha Base under heavy fire from Covenant armor and infantry.
Sergeant Forge and his Gauss Warthog crewmen head along the snowy road where they encounter a small group of Marines alongside their Warthog fighting a group of Needler wielding Elites. The Sergeant assists.
The surviving Marines mount on their Warthog and follow the Sergeant. After traveling across a broken bridge, they encounter more Marines.
Forge and his allies assist the group of Marines, and once clear the other group of Marines follow the Sarge onwards. Once again, another batch of Marine units is located ahead.
The Sergeant arrives and takes out the Shade Turret for the other groups of Marines.
The Marines continue on, meeting large amount of Unggoy.
As Sergeant Forge and the groups of Marines head closer to Alpha Base, more Marine allies await ahead under fire from more Covenant units. In the background, Covenant infantry erect an anti-vehicle barrier.
After Forge recruits the other units, they regroup near the entrance.
Marines enter the shield door while the Warthogs wait outside. Upon quickly routing the Kig-Yar infantry defending the power source, the marines open fire on the anti-vehicle barrier's power source.
Marines throw fragmentation grenades at the reactor, destroying it.
Multiple Shortswords fly overhead and a Pelican lands near the back of the base.