At the Harmony Grove picnic dedicated to abolishing slavery, Thoreau read portions of what became his essay “Slavery in Massachusetts” completed later that year.
The occasion was a Fourth of July celebration, and among Thoreau’s concerns was the fugitive Henry Burns who was extradited after escaping slavery and living in Boston.
Addressing the crowd, Thoreau said, “I wish my countrymen to consider, that whatever the human law may be, neither an individual nor a nation can ever commit the least act of injustice against the obscurest individual without having to pay the penalty for it. A government which deliberately enacts injustice, and persists in it, will at length even become the laughing-stock of the world.”
See http://thoreau.eserver.org/slavery.html – n2]; https://www.walden.org/what-we-do/library/lectures/thoreaus-lectures-before-walden-lecture-43/