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For this mission you take on the role of Sergeant Forge and his squad to evacuate the civilians of Arcadia from three Cargo Ships. Your starting resources are Four Hornets. Note that in this mission you are on the defensive but sometimes the polices will call for 'military help'. To earn The Real Winner achievement, save the civilian child named Adam.
Defend the Civilian Evacuees
You start off landing in Arcadia's Downtown area. The Arcadian Patrols will ask for support to clear out the Covenant guards trying to kill the civilians. Immediately have your units take out the nearby Covenant patrols before they can kill all of the civilians. Once you are clear of all enemies, the patrols will thank you. Then, a patrol will ask to your assistance for a Wraith problem. Then,you will notice the ship Cargo 3 attempt to evacuate, but in a scripted event it is destroyed by Covenant Banshees.
You will then have the opportunity to construct a base in a nearby area. Immediately take the opportunity to build one and construct a few supply pads, a barracks, a Vehicle Depot and an airpad, and train a good amount of units, but also keep in mind that ships Cargo 1 and 2 are still being loaded and attacked by the Covenant. Have a few marines to take out Jackals and Elites around the base and take their lookout towers.
Protect the Cargo Ships
Once you build a suitable army, send half of your units to Cargo 1 and the other half to Cargo 2. If the Covenant only attack one ship, have all of your units protect that ship.
For the allotted amount of time for the evacuation to begin, protect the 2 cargo ships and the civilians as they enter the ships. You have a few Spartan allies that will help protect you. Use them to your advantage, or just watch them as you can't really control them.
There is a Covenant Base just East of Cargo 1, very close to one of your base sites. It can be a really pain in the neck as it keeps spawning units. Build up a force of about 5 Wolverines and a few Scorpions and go take out that base. Use the Wolverines's Barrage ability to kill the base quickly, while using the Scorpions to kill enemy ground units (i.e. Wraiths). On higher difficulties, you may have to reinforce the Wolverines as they die pretty quickly, or you can start out with 10 Wolverines and Barrage the Citadel at the same time, killing it in about 15-20 seconds. After the Covenant base is dead, move the Wolverines to protect the Cargo ships. They are very good against Banshees.
Alternately, after you build your Firebase, go to lockdown mode. You should build a Reactor, then two hogs, and the rest Supply Pads. As soon as you can, jump back to your base and upgrade to gunner. After upgrading, build more hogs and protect Cargo 2 due to Cargo 1's proximity to the enemy base. Eventually, if things are toning down, upgrade to Fortress if you can and make an Airpad to supplement your Hornets, if any miraculously survived the onslaught. Build as many Supply Pads as you can and fully upgrade your hogs. DO NOT expand to two bases in this plan. This plan relies on luck and your ability to micromanage your hogs.
If one of the cargo ships happens to be destroyed, immediately transport all of your units to the other cargo ship. If you run low on units, train a few to replace them if your base still remains. If it is destroyed, ignore it as you still may be fine regardless.
Eventually, your base will get bombarded by a Mega Turret, but it should not kill your base unless you have neglected protection and prioritized the Cargo Ships. The Mega Turret is too heavily defended, so ignore it.
Once the Cargo Ships launch to evacuate the city, Sergeant Forge will bring the remaining units to the Arcadian Outskirts to patrol the area.