Civ Beyond Earth --- Can't conquer anymore cities

I can't seem to completely annihilate my enemy factions. I basically done beat the game having a 2000 score after fudging around end game (I ended with something like a 1500 score) but I can't conquer anymore capitals nevermind destroy a nation. Can you please explain what might be going on? I conquer quickly so I do have some cities under martial law. Is possible this is causing an issue? Could it be settings that I missed?

I greatly appreciate any help!

Solution 1:

You can only capture cities with a melee unit, not a ranged unit. Make sure you aren't just bombarding a city with ranged attacks, you have to have a melee unit go in at the end and seize control.