During his writing of "Civil Disobedience," Henry David Thoreau's biggest criticisms of government policy were of the institution of slavery and the Mexican-American War (1846–1848). He believed both were unjust and refused to pay taxes because he did not want his money used in support of either. For this act of resistance, Thoreau was arrested and spent a night in a jail cell. These two policies were the joint catalyst for him writing the essay "Civil Disobedience."
Thoreau was against slavery because he viewed the enslaved as human beings created in the image of God. He believed they had a right to freedom. Thoreau also stood against the Mexican-American War because he viewed the war was one of aggression on the part of the United States. It was even believed that the South, which fiercely supported the war effort, was hoping to expand slavery in the conquered territories, further intertwining these two injustices in the minds of abolitionists like Thoreau.
On the whole, these two policies had Thoreau's contempt because they were based on aggression, greed, and the domination of other human beings. For these reasons alone, he felt he had the right to defy the government in any peaceful way he could, as he ultimately argues in "Civil Disobedience."