Why did Achilles call the Greek leaders to an assembly?

During one of the Achaian (Greek) army's many raids on the cities located near Troy, the Achaians captured two beautiful enemy maidens, Chryseis and Briseis. Usurping Agamemnon's authority, Achilles calls an assembly of the army, and he suggests that a soothsayer be called upon to determine the cause of Apollo's anger.

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