How does the poem "Lights Out" by Edward Thomas relate to war?
You are doing a good job in reading in between the lines to ascertain that "Lights Out" by Edward Thomas can be about war! Many people simply think it is about sleep or, even deeper, death. But in regard to the latter, this poem can most certainly have to do with war, for war has much to do with death!
Take a look at Thomas's use of enjambment (the purposeful breaking up of lines to evoke a mood) to make us, as readers, feel out of touch and disoriented (just like we would feel before falling asleep...or dying). Take a look at this quote:
I have come to the borders of sleep,
The unfathomable deep
Forest where all must lose
Their way, however straight,
Or winding, soon or late;
They cannot choose.
Thomas, then, shows both sleep and death as absolutely unavoidable: "they cannot choose." And later in the poem, Thomas again writes that the "foliage lowers," which can be seen as nature enveloping the body once more... either of a civilian or a soldier!
Now also take the following lines:
Here ends in sleep that is sweeter
Than tasks most noble.
What could the "tasks most noble" be? Most definitely, the reader could take it to mean the noble task of being a soldier and fighting for the rights of others and/or for his/her country.
Therefore, the reader should feel free to take "Lights Out" by Edward Thomas as a description of the death of a soldier on the battlefield.