I thought it was: the fear and horror in the midst of war, and how it affects oneself, but my teacher said that was more of the topic. Help please!!
I would tend to agree with your teacher. The theme is more of what the author is saying about that topic. What is Owen saying about "the fear and horror in the midst of war"? How does it affect the young men or society? This poem has a very bitter, sardonic tone. In the end, the speaker claims that the age-old idea that it is sweet and glorious to die for one's country is a lie. He does this after very vividly describing the horrors he experienced during WWI. So, to think about the theme is to think about his point. What statement is Owen making through this poem, and who is the intended audience?
I hope this helps. Let me know if you have other questions.