Thoreau stopped paying his taxes to protest slavery and also spoke out against the Mexican-American War, which he believed was part of a Southern plot to expand slavery. After he criticized the war, he was jailed for nonpayment of his taxes. A relative paid his taxes, and Thoreau only spent one night in jail.
When he was released, Thoreau responded by writing his essay "Civil Disobedience." In it, he argues that citizens who believe in just government should withhold paying their taxes if their government acts unjustly. Since the US government was acting unjustly in saying it stood for freedom and liberty and yet legally allowing slavery, citizens of conscience were completely justified in committing civil disobedience.
In this essay, Thoreau asserts the important principle of primacy of conscience over primacy of law. Citizens have a duty, he argues, to stop a government that is perpetrating injustice. This duty is all the more imperative, he said, because governments by their nature tend to be corrupt and unjust. They need an active citizenry that will not only vote against injustice but act out against it.
Thoreau alludes to contemporary arguments about the "tyranny of the majority," which asserted that just because a majority of people vote for a law, that does not necessarily make the law right, sensible, or just. Because of the possibility that the majority could act immorally, it is up to the minority of those with a conscience to take a stand. In "Civil Disobedience" he writes:
A minority is powerless while it conforms to the majority; it is not even a minority then; but it is irresistible when it clogs by its whole weight. If the alternative is to keep all just men in prison, or give up war and slavery, the State will not hesitate which to choose.
Thoreau's concept of civil disobedience had a great influence on the civil rights movement of the 1950s and 1960s. A night in jail had a profound influence on the development of the American psyche.