[Suicide Junkie]

You got those 1200 troops into 800 space when you dropped 'em to attack.
Rather than vaporizing the excess troops, SE allows them to stay, though there may be penalties.
In SE3, for example leaving those excess troops on the planet reduced happiness.
Just load the troops back into the transports and move on to the next planet.

Look in SETTINGS.TXT to find how many millions of people are needed for each free militia.