I'd never seen this painting before today. Gassed by John Singer Sargent.

RP at work knew immediately this poem that is associated with it:

Dulce et Decorum est

Bent double, like old beggars under sacks,

Knock-kneed, coughing like hags, we cursed through sludge,

Till on the haunting flares we turned our backs,

And towards our distant rest began to trudge.

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 gas-shells dropping softly behind.

Gas! Gas! Quick, boys! An ecstasy of fumbling,

Fitting the clumsy helmets just in time,

But someone still was yelling out and stumbling

And floundering like a man in fire or lime.

Dim through the misty panes and thick green light,

As under a green sea, I saw him drowning.

In all my dreams, before my helpless sight,

He plunges at me, guttering, choking, drowning.

If in some smothering dreams, you too could pace

Behind the wagon that we flung him in.

And watch the white eyes writhing in his face,

His hanging face, like a devil's sick of sin;

If you could hear, at every jolt, the blood

Come gargling from the froth-corrupted lungs,

Obscene as cancer, bitter as the cud

Of vile, incurable sores on innocent tongues,

My friend, you would not tell with such high zest

To children ardent for some desperate glory,

The old Lie: Dulce et decorum est Pro patria mori. *

-- Wilfred Owen (1917)

* DULCE ET DECORUM EST - the first words of a Latin saying (taken from an ode by Horace). The words were widely understood and often quoted at the start of the First World War. They mean "It is sweet and right." The full saying ends the poem: Dulce et decorum est pro patria mori - it is sweet and right to die for your country. In other words, it is a wonderful and great honour to fight and die for your country

(From this site's annoted version of the poem- http://www.warpoetry.co.uk)