Final answer:
Henry David Thoreau believed that those who refused to fight in the war with Mexico were exercising their right to resist unjust government authority, a concept central to his philosophy of civil disobedience which advocates for nonviolent resistance to unjust laws.
Step-by-step explanation:
Henry David Thoreau was a staunch critic of the Mexican-American War and the institution of slavery. Thoreau's most significant contribution was his concept of civil disobedience, which he expounded in his essay 'Civil Disobedience'. Through his writing and actions, he advocated that individuals have not only the right but also the duty to resist government policies they find morally reprehensible. Thoreau executed this belief when he refused to pay his poll tax, an act for which he was imprisoned, due to its support for both slavery and the war with Mexico. In Thoreau's view, those who chose not to fight in the war against Mexico were exercising their right to resist authority, as he believed that the war was an unjust expansion of slavery's reach.
Thoreau's philosophy significantly influenced later figures like Mohandas Gandhi and Martin Luther King, Jr., emphasizing the power and necessity of nonviolent resistance to unjust laws. He was convinced that a just man's place, under an unjust government, might indeed be a prison cell if that was what it took to protest injustice. According to Thoreau, passive resistance was a method of nonviolent opposition to unjust authorities and remained a core strategy in numerous freedom struggles worldwide.