What did Helen have to do?

Legend has it that she was wife of King Menelaos of Sparta; she ran off with visiting Trojan prince Paris (Alexander); she compounded the problem by taking a lot of Menelaos' property with her. The Greeks under King Agamemnon of Argos mounted an expedition to recover her and, more importantly, the property. When the Greeks gained the upper hand, King Priam of Troy offered to give Helen back, but they demanded the property as well, and this being refused, persisted with the seige. After Paris was killed and Troy was captured, she was taken back to Sparta by her husband.

The reality is more like the Greeks (Achaians) organised a coordinated series of pirate raids on the coast and islands of Asia Minor. After an initial attempt on Troy, they pillaged the rest of the area over a period of about ten years, and then came back for another go at Troy, which succeeded. The bards wove this story and a lot of material from before and after it into an epic poem of which there were endless versions, each bard telling a different story. One of the most popular versions came from one known as Homer the blind poet of Chios, whose Iliad is about several weeks of the second attempt on Troy.