The poem begins with the poet transcending reality and reminiscing of one of his childhood memories. In this case it is one of when he as a child, watched a team of horses ploughing the stubble back into the field, during a rainy day which got progressively stormier. In the first two verses, the poet gives the reader a meaningful hint into what the circumstances of his times were. This was most probably, the hardships of a period of war. The few references Muir makes to an army such as in cases where the horses "marched" and the word "conquering" further strengthen this issue of war.
"Their hooves like pistons in an ancient mill"
This line brings up another issue which is plaguing the third world as we know it. In the same verse he refers to a "childish hour" in which he also compares the horses' hooves to pistons in an ancient mill. This refers to how child labour in factories was existent even then and how these dark memories were etched in his mind. We can suggest these memories to be dark not only by his imagination but by the "fearful" way he sees these images of the past.
Under the "great hulks" of these creatures he sees is however another truth. The way these symbols of "power" trod, allows the reader to infer another thought. Muir talks about...