Conquest of Planet Earth is an homage to every bad science fiction movie: bug-eyed aliens are trying to take over Earth, ostensibly because their Overlords said so. (Personally, I think it's just that Mars Needs Women, or possibly Mars Needs After-Dinner Mints.) You play as the aliens, crushing the Hunam resistance and conquering various bits of the planet.
During the game, various events can place tokens belonging to the human military on top of locations. Because the Random Number God hates you, those tokens can appear on top of places that you've already conquered, but don't have any units in.
What happens when a Hunam military token lands on a location you've conquered? I couldn't find any rules stating that the location is un-conquered. There's rules about starting a combat immediately if you have a unit there (which might kill the alien), but nothing about changing the state of conquest if you lose the combat. The battle rules make the implicit assumption that the aliens are invading the human space.
(As a side note, there are rules for Resistance Counters un-conquering Locations, but only in the Cooperative game where the aliens work together and the Hunam Resistance is weak. In the Competitive game, where the aliens are competing to get to 8 victory points first, there aren't any rules for this. The Cooperative rules are explicitly not part of the Competitive rules.)