The men are knock-kneed and coughing like hags because they are exhausted and battle weary from being at the front. They are "drunk with fatigue," yet they have to plunge on through sludge to get to their place of rest. Some have lost their boots and are walking on bloody feet.

As we find out, too, they are subjected to gas attacks, and though they have survived them, this could account, at least in part, for coughing like hags.

To be knock-kneed is often associated with being thin and weak. A hag is an old woman, and coughing is, of course, associated with illness. Owen chooses these particular images on purpose because his goal is to show that warfare in World War I is anything but glorious and heroic.

The title of the poem is in Latin and means "it is sweet and fitting" to die for one's country, a phrase that would have been familiar to his audience. Owen wants to show that this statement is a lie. War is not sweet, he argues, and it is not fitting to die for one's country.

Owen hopes that if people see the reality of war as it is, they will pay less attention to the propaganda and appeals to patriotism that convince young men that it is noble to enlist.

Why are the soldiers "knock-kneed and coughing like hags"?

The soldiers described in the poem are “knock-kneed and coughing like hags” because the conditions of war are so terribly hard on their bodies. Remember that soldiers would typically be young men, especially during World War I, likely just eighteen or nineteen years of age. This is why the speaker describes them as “children ardent for some desperate glory.” Despite their youth and their physical strength, these young men are “bent double” like old, decrepit people burdened by a heavy load. Their knees are weak from “trudg[ing]” through mud, some of them even without boots, and they cough like very poor, old women. They have likely inhaled all manner of dust and debris from the explosions of bombs, and their health would be very bad from lack of sleep, bad hygienic conditions, and less-than-nutritious food at the front.

Descriptions like this one help the speaker to support his claim, that it is, in fact, not at all sweet and becoming to die for one’s country, as suggested by the Latin phrase from which the poem takes its title. He depicts war as something that breaks down the health and spirits of even the healthiest young people. It degrades them, ages them, and ruins their physical and mental health. It is not, as some people would have us believe, glorious and sweet; rather, it is tragic and horrible.