Questions along this vein are usually fine, but the focus must be on the mythology not the history. Let me break that down with simple examples.
Suppose a Stack Exchange site is about Topic A. Anything "Topic A" is on-topic. Now suppose you have a question about how Topic A is related to Topic B. You can formulate a few questions:
- How, when, why did Topic A come from Topic B?
- How, when, why did Topic A affect Topic B?
These are good questions for TopicA.SE because the focus is on Topic A, not Topic B. Explanations:
How, when, why did Topic A come from Topic B? This question is asking about the origin of an item that falls under Topic A. From your example, the question might be "When did the myths about the Trojan War come about and what specific events from the war spawned the myths?" In this case, the myths about the Trojan War are Topic A, while the actual historical event of the Trojan War is Topic B.
How, when, why did Topic A affect Topic B? This question is asking about the way Topic A contributed to Topic B. The focus is on Topic A, and Topic B may very well have many other influences that do not fall under Topic A's domain. To follow your example, this would be like asking how a certain mythology affected an historical event. The full story of how that historical event precipitated is likely much more complicated than just how the mythology contributed to if, but the focus of the question is exclusively on how the mythology contributed. Your exact question is close to this, but is instead asking about the actual history not the mythology, so it is off-topic.
This is true for all topics. This site is about Mythology. Every question with two or more topics should focus on the mythology, not on the other topics. For example, on Christianity.SE a question might come in that talks about the relationship between Christianity and Judaism, but its focus is the Judaism, not Christianity; that's off-topic. Another example, History.SE may get a question that is about how the psychology of a certain people group was changed by a certain historical event, but the focus is on the psychology, not the history; that's off-topic.
Summary: All multi-scoped questions must focus on the the mythology aspect, not anything else.
- Mythology/History questions must be about the mythology, not the history.
- Mythology/Folklore questions must be about the mythology, not the folklore.
- Mythology/Anthropology questions must be about the mythology, not the Anthropology.
- Mythology/Psychology questions must be about the mythology, not the Psychology.
All questions must focus on Mythology, and not any secondary topics.