On July 24 or July 25, 1846, Thoreau ran into the local tax collector , Sam Staples, who asked him to pay six years of delinquent poll taxes . Thoreau refused because of his opposition to the Mexican–American War and slavery , and he spent a night in jail because of this refusal. The next day Thoreau was freed when someone, likely to have been his aunt, paid the tax, against his wishes.  The experience had a strong impact on Thoreau. In January and February 1848, he delivered lectures on "The Rights and Duties of the Individual in relation to Government",  explaining his tax resistance at the Concord Lyceum . Bronson Alcott attended the lecture, writing in his journal on January 26:
Thoreau also remained a devoted abolitionist until the end of his life. To support his cause, he wrote several works, including the 1854 essay "Slavery in Massachusetts." Thoreau also took a brave stand for Captain John Brown, a radical abolitionist who led an uprising against slavery in Virginia. He and his supporters raided a federal arsenal in Harpers Ferry to arm themselves in October 1859, but their plan was thwarted. An injured Brown was later convicted of treason and put to death for his crime. Thoreau rose to defend him with the speech "A Plea for Capt. John Brown," calling him "an angel of light" and "the bravest and humanest man in all the country."