War Museum Cambodia is a rough, open-air collection of tanks, artillery, aircraft, mines, and weapons from Cambodia's late-20th-century conflicts. It is not polished, but the lack of polish is part of the force: the exhibits sit close to the ground and close to living memory.
It makes a sobering contrast to Angkor's temple grandeur. Go with enough time to read and listen, because the value is not just in the hardware but in understanding how recent conflict shaped the country around the archaeological wonder most visitors came to see.