The poem opens in media res—in the middle of the action. The poem focuses on a single soldier who, as the title suggests, is in the middle of charging at the enemy with his bayonet (roughly setting the poem in the First World War). He is described as "suddenly" awake, which could mean that he had been sleeping during a lull in the battle.
That's pretty unlikely, of course; this is probably a metaphorical way of saying that the soldier suddenly becomes acutely aware of the horror and chaos around him like never before. This sudden awakening also foreshadows the soldier's epiphany that comes later in the poem, in which it dawns on him that patriotism is a hollow concept that means little in actual war—that he has, in some way, been duped.
The soldier is "heavy" with sweat and is described as "raw" (with that word repeated twice in quick succession). This relates to his physical exertion, but also to the intense fear that he feels as he enters the heat of battle. The intentionally awkward use of caesura and enjambment in the first two lines conveys the confusion and disorientating chaos of warfare:
Suddenly he awoke and was running – raw
In raw-seamed hot khaki, his sweat heavy,
Lines 3 and 4 intensify the sensory overload of the battlefield. The thick consonance in line 3 feels strained and cumbersome, evoking the difficult terrain over which the soldier makes his charge:
Stumbling across a field of clods towards a green hedge
Rifle fire "dazzle[s]" the soldier's peripheral vision, building a sense of immediate threat. The poem uses an intentionally weird metaphor here as well, describing the gunfire as "smacking the belly out of the air." This metaphor doesn't quite seem to make sense—the air doesn't have anything like a belly; instead, this points to the intense force of the bullets as they fly past.
The image, combined with the /b/ alliteration ("Bullets" and "belly"), is unquestionably violent, and suggests that the soldier is lucky for every additional moment he avoids being hit. The end-stop here, executed with an m-dash after the word "air," also looks like a blast of horizontal rifle fire and interrupts the flow of the poem.