Final answer:
Siegfried Sassoon's 'Counter-Attack' portrays the physical and emotional descent of soldiers in a failed counter-attack during WWI, using vivid imagery to highlight their bravery and the harsh conditions of trench warfare.
Step-by-step explanation:
The excerpt from Siegfried Sassoon's poem "Counter-Attack" depicts the gruesome reality of a failed counter-attack during World War I. Sassoon uses vivid imagery to convey the chaos and horror of battle, as seen through the eyes of a soldier. We read about machine guns, bombing, and the mud-clotted bodies of fallen comrades. As the poem progresses, the soldier ultimately succumbs to his wounds, sinking 'down, and down, and down' in a metaphorical and physical sense. The soldiers mentioned in the question went 'down so tall in the onslaught' because they faced heavy enemy fire and physical exhaustion, fighting bravely until they were overpowered and defeated in the brutal reality of trench warfare.