The event that marked the beginning of protests ultimately leading to the American Civil War was the Fugitive Slave Act. Enacted in 1850, it mandated the return of runaway slaves to their owners, igniting outrage in the northern states. Abolitionists and free blacks rallied against this legislation, emphasizing the moral implications of slavery. The Act heightened sectional tensions and mobilized public sentiment against the institution of slavery, escalating confrontations between pro-slavery and anti-slavery factions. The growing discontent would culminate in the secession of southern states and trigger the Civil War.