Given the history of humanity, war on the moon wouldn't be that surprising. We've fought everything else, but now the moon's south pole, where frozen water hides in permanently shadowed craters, is shaping up to be the next contest.
The U.S., China, and India all have active lunar programs aimed at putting humans in roughly the same neighborhood. They are looking for water and it isn't just for drinking. Split it into hydrogen and oxygen and you've got rocket fuel, which makes the moon a potential gas station for deeper space. Strategic resources plus overlapping claims is a familiar recipe for war.
The 1967 Outer Space Treaty bans nuclear weapons in space and military bases on the moon, but it's silent on conventional weapons. Firearms would work on the moon. Bullets would travel much farther in low gravity with no atmosphere to slow them down. Lasers would thrive in the vacuum.
Robot war?
If war ever reaches the moon, humans probably won't do the fighting. Robots will.
Imagine the Sci-Fi image of dust-caked machines trudging across craters a century after the humans back home signed a peace deal. But, it's also possible that two rival robots meeting at a contested ice deposit could, in principle, compare notes and realize their objectives don't actually conflict.
Whoever builds the robot decides the future. Militaries have strong incentives to build AI smart enough to win fights but not smart enough to question them. A robot that can refuse a stupid order can also refuse a smart one its commander really wants carried out.
