Henry David Thoreau (1817-62) was an author, essayist, naturalist, and poet whose work went on to influence some of the greatest literary figures of the twentieth century. Thoreau expounded upon his love of nature and the doctrines of Transcendentalism in Walden (1854), and passionately defended civil liberties and pacifistic protest in the essay "Civil Disobedience" (1849). His friend and mentor was Ralph Waldo Emerson, one of the most prominent intellectuals of the nineteenth century.Portrait by Benjamin D. Maxham (daguerreotype), black and white of Henry David Thoreau in June 1856. Wikipedia, The Free Encyclopedia.
Thoreau's essay, "Civil Disobedience," emerged out of his experience opposing the Mexican-American War. He had refused to pay a poll tax as a demonstration against what he felt to be an unjust, imperialistic war and a government waging it to expand slavery's domain. He spent the night in jail (July 23rd 1846) before someone paid the tax to set him free. In his essay, Thoreau argued that not all civil laws are just, and that humans have an obligation to obey a higher law—their sense of morality. If obeying the conscience necessitates violating the law, then so be it. Thoreau advocated that others who disapprove of the war follow his lead and refuse to pay their taxes as a gesture of protest. "Civil Disobedience" received little notice at the time it was written but enjoyed a revival in the twentieth century with the self-determination movement of Mahatma Gandhi and the Civil Rights Movement of Martin Luther King, Jr.
You can see more about the Mexican-American War in: http://www.pbs.org/kera/usmexicanwar/timeline_flash.html