I've been toying with this idea for a while now, it's by no means fleshed out yet but here we go:
So there's this featureless white plane. It stretches on infinitely in every dimension and is made of an unbreakable, impenetrable material. Situated above this is an equally large ceiling with a massive, gaping hole in it that almost reaches the horizon. No matter how high you go the ceiling never gets any closer, and no matter how far away you run the hole's always perfectly above you. This basically means that without another object to compare to there's no real sense of direction or distance.
Good thing there's objects, then! Random things, from teddy bears to oak trees to alien moons, randomly fall out of the hole and land in random locations around this world. They're pretty spaced out, though, and after some time left untouched "melt" back into the floor so there's always lots of empty space. Sometimes animals (some from Earth, some not) fall down too, and while most perish the humans and aliens that live tend to band together and form massive forts made of salvaged material. Since there's no curvature or landmarks to cover your view, these exist in plain view and everyone miles around can see their activities.
Finally onto the question: Let's say one of these forts wanted to attack their neighbor. They want to send an army to invade, however, the enemy would plainly be able to see the approaching soldiers against the white and prepare accordingly. If they theoretically had infinite resources of all types from the hole, and all the time in the world to prepare, what can they set up to hide their army's location or otherwise maintain surprise?