The thesis should be the main idea of the essay in one sentence-the idea your essay will seek to proof. Your essay will be summarizing and responding to your chosen WWI poem. In this case your thesis should involve your own response to your chosen poem. Avoid statements such as I think and instead seek to prove an idea about the poem. 
Poem:
https://www.poetryfoundation.org/poems/57322/for-the-fallen
For the Fallen by Laurence Binyon