A line in long array where they wind betwixt green islands,
They take a serpentine course, their arms flash in the sun—hark to
 the musical clank,
Behold the silvery river, in it the splashing horses loitering stop
 to drink,
Behold the brown-faced men, each group, each person a picture, the
The author of the poem creates almost a motion picture or slideshow of images in the reader’s head, as to capture the moment, by using adjectives and verbs to describe specific details of the moment that appeal to our senses.  For example, “their arms flash in sun” creates an image of bright color by using the verb “flash.”  Also, The excerpt “the musical clank” creates a beat that the troops might march to, or that the reader might read the poem to.  This particular excerpt appeals to the sense of sound.
Cavalry Crossing a Ford