1. Agamemnon probably used his sister-in-law's defection as a pretense to attack Troy, which he had been wanting to do. As the ruler of many countries, it gave him a reason to force the kings under him to help him, and most seemed highly reluctant to help.
Helen was a figurehead. But in some ancient texts, Helen of Troy was also a fake. The story goes that when Paris and Helen stole away from Melelaus's castle they stopped first in Egypt, where an Egyptian priest stole the original Helen and sent Paris off with a simulacrum. The real Helen never got to Troy. The long war was fought over a fake Helen and the fake Helen went home to live with Menelaus. (The fakeness of Helen might explain some oddities in Book IV of the Odyssey, when Telemachus goes to Menelaus's court and meets Helen... why Helen was never able to conceive another child, although she had previously borne Hermione; why, in Homer's Odyssey, she seems so content to be back with Menelaus; why the Egyptians figure so prominently in post-Troy-Helen's household.) The biggest war in history was fought on a pretense, and even the pretender did not know that his pretense was a fake.
2. During the 20 years of the Trojan war, kings all over the Greek world were cuckolded by wives that were left alone at home. Some, like Agamemnon's wife Clytemnestra, took up with another man of their own accord. Some, like Odysseus's wife Penelope, had power-hungry suitors trying to force themselves on them. You could say that in avenging a man whose wife cheated on him, the avengers created a situation where their own wives cheated on them.
The cuckolding had huge ramifications... the great King Agamemnon, king of all the Greeks, fresh from his victory over the powerful Trojans, came home and was murdered by his wife Clytemnestra and her lover Aegisthus (at least according to Aeschylus in his Oresteia).
3. At the very end of the Iliad, Helen says of Hector, "I've never heard a nasty word from you or an abusive speech" and yet in Book III, Hector accuses Paris of being a "woman-mad seducer." In Book IV of the Odyssey, when Helen is back home with Menelaus, he refers to her as "my dear wife" and pampers her. Even though in the Iliad Helen describes herself as a "horrible, conniving bitch," in the Odyssey Helen blames Venus for "taking me over there, away from my country, my girl, and my lawful wedded husband." You would think that even if she were a pretense, she would be blamed by the central characters for providing a pretense.
The only people who seem to dislike Helen are the citizens of Troy (at the end of the Iliad, Helen says that Trojans "all look at me and shudder with disgust") and Achilles (in Book XIX, he says he detests her). But even in those cases, there is no direct allegation of blame.
Paris has a pretty big role in the Iliad, but Helen is oddly absent. (She gets more coverage in the Odyssey, where she is just a matron entertaining a guest in a side-story.) The Iliad is about the heroes Achilles and Hector, and the woman who supposedly started it all is barely mentioned. At some points in the Iliad it seems that Menelaus is more concerned with some unspecified goods that Paris stole than with Helen, but that is mentioned only in passing.
Helen isn't portrayed as stupid or even passive... she is just somehow completely unaccountable. Homer assigns more blame to a storm that wrecks a ship than he does to the woman who caused the biggest war and arguably the largest slaughter of men that had ever occurred. As in the simulacrum version, Helen seems to be missing.
* Roberto Calasso, The Marriage of Cadmus and Harmony