Owen makes some graphic comparisons regarding the soldiers that help readers understand the emotional and physical realities facing the men who fought in World War I. First he compares them to "old beggars under sacks." The soldiers were like beggars because they were "bent double" under the weight of their military packs that they carried in the way a homeless person, who carries all he owns in a lumpy sack with him wherever he goes, crouches under the weight of his burden. The men were, in a sense, homeless in that they had to move from camp to camp wherever their battle orders took them. This stands in stark contrast to the way the jingoistic war poetry of the day portrayed the life of a soldier. Such poems, which Owen was writing in response to, and other recruiting materials made the war seem like an elite country club (see the poster "The Army Isn't All Work"; source: WWNorton.com). Owen dispels that image with this comparison.
Next, he compares the soldiers to "coughing ... hags," another very unflattering portrait. The damp and dirty conditions in the trenches caused a variety health problems for the soldiers. According to one source, many of the soldiers had either influenza or "pyrexia," a category that included various fevers. With the unsanitary conditions and being knee-deep in mud (the "sludge" referred to in line 2) most of the time, being sick was common, but men had to fight on. Again, the point of the comparison is to show the reality of war rather than the glamorous way it was often depicted by recruiters.
Other comparisons are made through hyperbole: "men marched asleep," "all went lame; all blind; drunk with fatigue." The emotional and mental numbness and disorientation the men endured is suggested by comparing them to men who are sleeping or who drink too much. This comparison conveys the idea that the suffering was not just physical; it was also psychological. Although the men were literally lame, many from trench foot or inflamed muscles, the word brings to mind those who are disabled from losing a limb or from permanent injury. The men still fighting hadn't reached that point yet, but it was a distinct possibility for them. Comparing the soldiers to blind men again reveals their mental as well as their physical state: they were so tired they couldn't see straight, as the saying goes, with sight standing for mental cognition as well as vision.
Owen's comparisons graphically depict the physical and mental hardships the soldiers went through and starkly contrast with the unrealistic portrayals of the war in the recruiting materials of his day.