The final mission within the main quest - the objective is to reclaim the purifier. It is started by reaching the citadel and speaking with Elder Lyons in the lab.
You may be offered an honorary position within Pride, if this happens, you have a choice of receiving Power or Recon Armor, either of which is at full condition.
Once the attack starts, heat out of the courtyard, and follow the giant robot. The robot in question has a powerful beam that can destroy the vertical aircraft and convert the enemy "communists" into swiss cheese. It also has a supply of explosive cases that it uses as powerful grenades. To avoid too much health loss, follow the robot at a slight distance but not too far away from the action.
While the robot is approaching the purifier, it will be shelled by mortars. The launcher is too far away to deal with, and it is also behind the energy fields. The mortars are not blocked by terrain, thus you can't take cover behind a building. Furthermore, these mortars will cause cars to explode.
The robot will finally reach the Jefferson Memorial, opening up the final quest.