Interestingly, the verse he quotes and calls a lie is by Horace, who was himself a combat veteran. Horace was an officer in Brutus’s army, and served on the losing side at the battle of Philippi. So, while he didn’t give his life for his country, he persumably knew something of the horrors of (non-modern) war.
Speaking of which, got this from Wikipedia:
“After the assassination of Julius Caesar, Horace joined the army, serving under the generalship of Brutus. He fought as a staff officer (tribunus militum) in the Battle of Philippi. Alluding to famous literary models, he later claimed that he saved himself by throwing away his shield and fleeing.”
Never heard it before, but if true it seems like it kinda takes the “oomph” out of his later line.
[shrug] Does pro patria mori even apply in a civil war?
I’d say so. You’re the one fighting for your county, they’re the godless rebels who would see all it stands for rent asunder, yadda yadda yadda, whoever the yous and thems may be.
On the other hand, that does present a compelling loophole: “Hey Horace, about that whole dulce et decorum thing, didn’t you drop you shield and run from the Battle of Phillipi?” "Yeah, but see, that was a civil war and we were technically dying for the same country, so I figured “You know what? Fuck this!”
John McCrae, the author of In Flanders Fields didn’t see combat directly because he was a surgeon, but he certainly saw enough death from the war. Does that count?
Totally OT, but I thought it was strange that none of the photos on that page showed a ball turret. The top photo was taken from an angle where the landing gear hid the turret, the middle photo was of a nose turret of a British bomber, and the bottom photo showed the remotely controlled chin turret of the B-17G.
Robert W. Service’s Rhymes of a Red Cross Man* are poems inspired by his service in WWI as an ambulance driver. I don’t think they’re nearly as well-known, or revered as the Wilfred Owens pieces linked above, but they’re still pretty heady stuff. I like the collection, as a whole, too - because the individual pieces run a gamut, from incredibly jingoistic, such as Jean Desprez; to the anti-war screed The Stretcher-Bearer; the stresses associated with simply manning a modern war, A Song of Winter Weather; and touches, too, on the stresses of the returning veteran with Fleurette, and Afternoon Tea.
- If one has an interest in reading the collection in the order that the author intended - that site has things set up so that the last poem in the book is the first listed, and then works backwards - so start with the last poem on the last page of the listing linked here.
I rushed in here to defend Wilfred Owen’s honor, and I see I’ve been beaten to it many times over. I’m glad. 
He was brilliant. And I second jjimm’s recommendation of Pat Barker’s WWI trilogy, although Owen only appears in the first book (IIRC).
Other great poems of his:
Exposure
Parable of the Old Man and the Young
Greater Love
Insensibility
Brian Turner-Here, Bullet. Dealing with his time in Iraq.
“Ashbah”
The ghosts of American soldiers
wander the streets of Balad by night,
unsure of their way home, exhausted,
the desert wind blowing trash
down the narrow alleys as a voice
sounds from the minaret, a soulfull call
reminding them how alone they are,
how lost. And the Iraqi dead,
they watch in silence from rooftops
as date palms line the shore in silhouette,
leaning toward Mecca when the dawn wind blows.
Robert D. Wilson’s Vietnam Ruminations (PDF).
A simple soldier recounting his experiences in America’s war and relating in the universality of haiku experience. Robert D. Wilson is a truly excellent haijin, and I have had the pleasure of his correspondence and teachings.
Joh Ciardi was a air corps gunner in the Pacific Theater of WWII, and wrote a book of poems, Other Skies about it. Are they good poems? I don’t know. Was he a good shot? I don’t know that, either.
*No fire-shot cloud pursued us going home.
No cities cringed and wallowed in the flame.
Far out to sea a blank millennium
Changed us alive, and left us still the same. *
V-J Day
Perhaps High Flight by John Gillespie Magee Jr. deserves a mention. It isn’t really battle poetry, though it was inspired by combat training.