Final answer:
In Henry David Thoreau's excerpt, the rhetorical device used is a metaphor, where he compares men to tools in the hands of the powerful.
Step-by-step explanation:
In the quote from Henry David Thoreau's Resistance to Civil Government, "Now, what are they? Men at all? or small movable forts and magazines, at the service of some unscrupulous man in power?", Thoreau uses metaphor as the rhetorical device. A metaphor is a figure of speech that describes an object or action in a way that isn’t literally true, but helps explain an idea or make a comparison. Here, Thoreau metaphorically compares men to "small movable forts and magazines", implying that they are simply tools at the disposal of those in power, rather than independent beings with their own will.
Thoreau's works Civil Disobedience and Walden argue for individual freedoms against government interference and societal expectations, influencing later civil rights leaders with the concept of passive resistance, or nonviolent protest, against unjust laws.