Sonnet V: The Soldier by Rupert Brooke. Brooke died in 1915 of blood poisoning due to a small wound, left unattended.
“If I should die, think only this of me:
That there’s some corner of a foreign field
That is for ever England.”

In Flanders Fields by John McCrae.
“If ye break faith with us who die
We shall not sleep, though poppies grow
In Flanders fields.”

As the war dragged on, men became disillusioned, and the poetry became darker and more pessimistic.

Dulce et Decorum by Wilfred Owen. Listen to this poem by a British soldier who was killed in action in 1918 a week before the war ended.
“Men marched asleep. Many had lost their boots
But limped on, blood-shod. All went lame; all blind;
Drunk with fatigue; deaf even to the hoots
Of disappointed shells that dropped behind.”

Suicide in the Trenches by Siegfried Sassoon.
I knew a simple soldier boy
Who grinned at life in empty joy,
Slept soundly through the lonesome dark,
And whistled early with the lark.

American Alan Seeger foretold his own death in the poem, Rendezvous.
“I have a rendezvous with Death
At some disputed barricade . . .”

This Is No Case of Petty Right or Wrong by Edward Thomas.
“I hate not Germans, nor grow hot
With love of Englishmen, to please newspapers.”