The tone in 'Dulce Et Decorum Est' has been built in many ways, most importantly through fear, death and disillusionment. Owen has used poetic and literary effects to show that war is bad.
Everyone has fear of heights or something, but that seems nothing to what these soldiers were experiencing. The section, 'Men marched asleep. Many had lost their boots/ But still limped on, blood-shod. All went lame; all blind/ Drunk with fatigue.' (1st stanza) not only lets us use this picture the soldiers, but it helps us understand the poor conditions and shows us that not only were they tired, weak and dirty, but scared. He also did this by using metaphors - 'As under a green sea I saw him drowning.' (2nd stanza). This helped me understand that he felt trapped under this gas, scared.
When Owen started describing the injured soldiers he gets straight to the point and captured the theme of death by using 'ghastly' words to describe jus how bad it was - 'His hanging face, like a devils sick of sin.'
Using disillusionment puts a whole different view on the poem. It helps us realise that these solders were tricked into dying for their country. They lost their lives to a lie.
By using many imagery phrases, similes and metaphors, Owen has clearly shown the tone of the poem.