Why did Artemis make Agamemnon kill his own daughter?

Any summary of Agamemnon dilutes the complexity, ambiguity, darkness, and confusion of the play, as well as its power and beauty. It is useful to note, however, that the chorus of Argive elders makes it clear that Zeus himself commanded the two sons of Atreus, Agamemnon and Menelaus, kings of Argos, to avenge the seduction of Helen, Menelaus's wife, and her being carried off to Troy. (In Homer, Menelaus is king of Athens's new enemy, Sparta, while Agamemnon is king of Mycenae, a city destroyed by Athens's new ally, Argos.) When the army gathers at Aulis on Euboea in a great fleet of ships to sail to Troy, contrary winds impede its departure. Artemis, the virgin goddess of the hunt, is angry at the death of the innocent that war will inevitably entail. Or is she angry at the death of the young of the pregnant hare killed by the twin eagles in the omen that foretells the Greek victory? Whatever the case, she commands that Iphigenia, daughter of Agamemnon, be sacrificed before the winds will become favorable for the fleet's departure.