Tone in literature is the attitude or feelings a writer expresses towards the subject at hand. Tone is expressed primarily through word choice. For example, when Thoreau opens his essay with the words "heartily approve" in reference to small government, we know that he is invested emotionally in his topic, rather than simply recording facts as a neutral observer. We can, therefore, state from the start that Thoreau's tone is passionate rather than detached.
Thoreau, who has been put in jail for not paying his taxes, takes a negative and indignant tone towards government power. He makes it clear that he believes that the least influential and smallest government is the best as he lashes out first against government in general and then against what he believes are the government-sanctioned tyrannies of the Mexican American war and slavery. He states, for example, that, in general, "It [government] does not keep the country free." He states too that
I ask for, not at once no government, but at once a better government. Let every man make known what kind of government would command his respect, and that will be one step toward obtaining it.
The two quotes above show another way through which Thoreau expresses tone; he does so by italicizing words to add emphasis to them. When he repeatedly italicizes that government is an "it"—a thing, an abstraction, not a person—he is signaling and emphasizing his disdain and indignation at it from a superior position. When he italicizes "at once," he is putting emotional urgency into the idea of installing a better government immediately.
Thoreau is trying to build a logical case for a small, moral, and ethical government in this essay. However, along with logic, he adopts a passionate tone that convinces his audience of his emotional investment in the subject.