You might first want to do some research comparing the outline of Homer's The Iliad to the movie Troy. We know, for example, that the directors of the movie went to great lengths in order to depict Bronze Age warfare accurately--armor, weapons, and tactics. Although they recreated the setting, it's likely that the historical Troy was much smaller and less imposing than the Troy depicted in the movie.
As you discover the differences between Homer's Iliad and Troy, you will be able to develop a thesis in which you either argue that the two have more similarities than differences or, alternatively, that the movie took many liberties with Homer's story for the sake of dramatic effect. This effort might lead you to write a comparison/contrast research essay in which you point out and discuss the differences between the history of the war and the movie treatment of that war.
Among other aspects, researching Homer's Iliad will point you to the differences and similarities between the chief characters in the story, and that my be a sufficiently large subject for the research essay--in other words, focus on the characters involved and how they appear in history and the movie rather than on the entire story. For example, how do the Iliad and Troy depict the relationship between Achilles and Agamemnon? What is the role of the gods in both works? There are several interesting subjects you can discuss in these two works that would yield an interesting research essay.
The thesis you develop will be a result of your research, but it may be something like this: "Although Homer's Iliad and the movie Troy treat the same historical event, many important differences exist in their treatment of historical details and important characters in the story." This example is meant to help you think about how to construct a thesis along the lines I've suggested, but, again, your thesis will reflect what you find during your research.