Based on Thoreau’s beliefs and "Civil Disobedience," he most likely decided not to pay poll tax because

Thoreau believed people should refuse to support a government when it acted unjustly. In “Civil Disobedience,” he explains that he withheld his poll tax as a protest against the government’s support of slavery and the Mexican-American War. So he refused to pay because he disagreed with what the tax supported, not because he could not afford it or did not understand the law.

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