The event that catalyzed the abolitionist movement in the United States was the publication of "Uncle Tom's Cabin" by Harriet Beecher Stowe in 1852. This influential novel exposed the brutal realities of slavery and humanized the plight of African Americans, igniting widespread public outrage and empathy. It served to galvanize both abolitionists and the general public against the institution of slavery, making the moral arguments more accessible. The book's significant impact contributed to increasing anti-slavery sentiments in the North, played a vital role in the Underground Railroad, and intensified national debates ultimately leading to the Civil War.