The poem dismisses the so-called patriotic fervor and glory of war absolutely and the poet clearly expresses his cynicism for this glorified method of stating and establishing one's power and supremacy.
From the outset, Owen confronts the reader with the true, and indeed negative reality, of actual warfare. His perspective is that of a participant on the front, actually involved in the fight, and not of some armchair onlooker comfortably commenting on the events he is witnessing. This adds to the realism of the poem and makes what the speaker says so much more powerful.
It is pertinently clear that the speaker is disgusted and traumatised by what is happening around him. He uses powerful images to describe the men's suffering:
Men marched asleep.
The soldiers are exhausted but plod on because they are forced to. Stopping would mean either death or a court-martial. They therefore have no choice.
... Many had lost their boots
But limped on, blood-shod. All went lame; all blind;
Drunk with fatigue; deaf even to the hoots
Of tired, outstripped Five-Nines that dropped behind.
So overwhelmed are the men by exhaustion that they are deaf and blind to the exploding shells around them. Their fatigue puts them in a drunken, hallucinatory state. They are badly injured, but continue their march. Owen uses short, powerfully descriptive phrases to emphasise the terrible conditions that the men find themselves in. No-one is excluded - they all face the same terrible hardship.
The second stanza reveals greater atrocity. The soldiers had to, as tired as they were, protect themselves against the noxious gases with which they were attacked. They struggle to put on their gas masks:
Fitting the clumsy helmets just in time;
But someone still was yelling out and stumbling
And flound’ring 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.
The speaker witnesses a fellow combatant 'drowning' as if he were in the water. He could not breathe for he could not get his mask on in time. He was overwhelmed by the terrible fumes and the simile, 'flound’ring like a man in fire or lime...' is a gruesome description of his agonising struggle, even his death.
So terrible was the image that it stayed with the speaker, haunting him in all his dreams:
In all my dreams, before my helpless sight,
He plunges at me, guttering, choking, drowning.
The speaker paints a poignant picture for he was helpless to assist his comrade and could but only witness his desperate struggle.
The speaker extends the image of his comrade's desperate and tragic struggle in the next stanza and places the reader in his position.
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,—
He vividly illustrates, through pungent metaphors and ghastly similes the horror that both he and his dying mate experienced. He concludes that, if the reader should have been present, and overwhelmed by stifling dreams of this particular incident, he/she would certainly not have agreed that, 'It is good and honourable to die for ones country.'
It is a lie.