Antislavery campaigns developed during the nation's founding period. Pennsylvania Quakers were active abolitionists who believed slavery violated their religious values and contradicted fundamental principles of liberty and equality. For nearly a century, civic and religious antislavery associations regularly petitioned Congress to prohibit slavery. Although Congress banned the importation of slaves in 1808, the institution of slavery did not end until after the Civil War with the adoption of the 13th Amendment in December 1865.