Despoil is to spoil, only worse. You may spoil a dinner party by being late, but we all despoil the earth with pollution and over-consumption.
The Latin root of despoil meant "to strip or rob" and although despoil is used more broadly now, it has that sense within it. When we despoil the earth, we rob it of its beauty and strip it of its natural resources. In the same way, an invading army might despoil a village, wrecking houses and stealing valuables.