Wilfred Owen, like so many other people, was deeply affected by the experience of World War I. In previous wars, the armies would line up and charge one another. In WWI, however, chaos and destruction were the only rules.
In "Dulce et Decorum Est," Owen decribes what it was like for men who had been attacked with mustard gas; it is not made from mustard but got its name because of its odor. This gas attacks the lungs:
Exposure to low concentrations of mustard gas classically causes the reddening and blistering of skin and epithelial tissue. On inhalation, the gas causes the lining of the lungs to blister and leads to chronic respiratory impairment. Higher concentrations of mustard gas will attack the corneas of the eyes and can cause blindness.
When the soldiers in the poem detect the gas, the scramble to put on their gas masks. All but one manage to do so. Owen describes how that lone soldier dies in agony.
Masterplots states the author's purpose well:
That memory prompts the narrator to offer in the final verse paragraph some bitter advice to readers about the nature of warfare and the outcome of blind patriotism. In the last twelve lines of the poem, Owen describes his experience of walking behind the wagon in which the dead man has been placed, seeing the corpse frozen in the twisted agony of its death throes. That sight, he says, would prevent any man from adopting glibly the notion that dying for one’s country is somehow noble.