A more thorough explanation:

Definition: To deprive a person of possessions illegally by violence or by clandestine means; to rob.

Examples:

• The invaders despoiled the village, taking everything of value.
• The thieves despoiled the museum, stealing priceless artifacts.

These examples illustrate how despoil means to take something by force or illegally. In the first example, the invaders took everything of value from the village, which was not theirs to take. In the second example, the thieves stole priceless artifacts from the museum, which is illegal and deprives others of the opportunity to enjoy and learn from them.