Fate is a very powerful force in Homer's Iliad. As in all Greek epics, the role of the gods is paramount in the Iliad, and it is difficult to separate their will from the idea of fate more generally. Some argue that the fate of an individual is beyond even the gods to affect. Others suggest that it can be altered by a specific god's will.
Whatever the case, it is clear that fate is as capricious and unpredictable as it is unavoidable. The Iliad's main focus is the events of the Trojan War, a war which begins as a result of the Golden Apple of Discord. Zeus gives the Golden Apple to Eris, the goddess of strife, to create chaos. And chaos it creates. In fighting over who is the most beautiful and deserves the Golden Apple, Aphrodite, Hera, and Athena each promise Paris something in particular. He ends up being particularly drawn to the idea of Helen of Troy falling in love with him. It is this that precipitates the war, since Helen is already married to King Menelaus.
Throughout the conflict, various figures make all sorts of decisions, but it is clear that no matter what they do, they can't avoid their fate. Their character comes to be a significant factor in determining their fate. One thinks here of Achilles's wrath.