The mission I'm talking about is only accessible through the mission select screen:
This is how it looks when you start the mission:
This is where you constantly get reinforcements:
As you can see a plane is just arriving with reinforcements which include troopers and trikes and/or quads.
This is where the starport is with a small base of a brown player:
In this screenshot I've already captured the starport as well as 2 wind farms and a refinery. I built the barracks myself. This part of the base also included quite a few more wind farms as well as a heavy factory and had like 7-8 gun turrets. The brown player also has another base to the west just above the explored area. It has an outpost a refinery some gun turrets, and unit production buildings. Above that base in the upper left corner is a small Harkonnen base. The objective of the mission is to capture the outpost of the brown player that is below the Harkonnen base.