Final answer:
Henry David Thoreau's metaphor indicates that the army is an integral part of the government, essential to its function, operating under governmental control.
Step-by-step explanation:
Henry David Thoreau's metaphor, "The standing army is only an arm of the standing government," means that the army is essentially an extension or component of the government itself, suggesting that it's one part that plays a role within the larger body of governance. Thoreau uses this metaphor to highlight the integral relationship between the military and the government, indicating that the army operates under the direction and control of the government, rather than being an independent or inconsequential force. This understanding aligns with the larger themes in Thoreau's work, which often critique government actions and emphasize the potential for individual resistance against unjust authority.