My mother (born 1919) remembered reciting "In Flanders Fields" each Armistice Day in her little school in Somerville.
I taught this poem in senior English for many years (along with all the anti-war poems by Owen, Sassoon, Rosenberg, etc.)
I assume we all know this, but just to say it, the conceit used in many of the WWI poems about the poppies was that these ubiquitous flowers were red because they were nourished on the blood of the fallen. For example, Isaac Rosenberg's "Break of Day in the Trenches":
The darkness crumbles away.
It is the same old druid Time as ever,
Only a live thing leaps my hand,
A queer sardonic rat,
As I pull the parapet's poppy
To stick behind my ear.
Droll rat, they would shoot you if they knew
Your cosmopolitan sympathies,
Now you have touched this English hand
You will do the same to a German
Soon, no doubt, if it be your pleasure
To cross the sleeping green between.
It seems you inwardly grin as you pass
Strong eyes, fine limbs, haughty athletes,
Less chanced than you for life,
Bonds to the whims of murder,
Sprawled in the bowels of the earth,
The torn fields of France.
What do you see in our eyes
At the shrieking iron and flame
Hurled through still heavens?
What quaver -what heart aghast?
Poppies whose roots are in men's veins
Drop, and are ever dropping;
But mine in my ear is safe,
Just a little white with the dust.
An excellent book that analyzes the war from a cultural perspective is Paul Fussell's The Great War and Modern Memory. (Fussell was a front-line infantry lieutenant in WWII, and wrote a couple of books about that.)
One of his major points is that the famous anti-war poetry from WWI (almost always written by combatants who did not survive, Sassoon being an exception) was essentially rediscovered and "invented" as a genre during the anti-Vietnam War protests of the 60s. During the war itself, patriotic poetry such as the one in the OP and "If I should die, think only this of me,/That there's some corner of a foreign field/That is forever England" helped give meaning and nobility to the incredible suffering and carnage.