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Civil War and Reconstruction Era
Further information: American Civil War and Reconstruction Era
The Battle of Gettysburg,Pennsylvania during the Civil War byThure de Thulstrup.
Differences of opinion and social order between northern and southern states in early United States society, particularly regarding Black slavery, ultimately led the U.S. into the American Civil War.^[100] Initially, states entering the Union alternated between slave and free states, keeping a sectional balance in the Senate, while free states outstripped slave states in population and in the House of Representatives. But with additional western territory and more free-soil states, tensions between slave and free states mounted with arguments over federalism and disposition of the territories, whether and how to expand or restrict slavery.^[101]
With the 1860 election of Abraham Lincoln, the first president from the largely anti-slavery Republican Party, conventions in thirteen slave states ultimately declared secession and formed the Confederate States of America, while the U.S. government maintained that secession was illegal.^[101] The ensuing war was at first for Union, then after 1863 as casualties mounted and Lincoln delivered his Emancipation Proclamation, a second war aim became abolition of slavery. The war remains the deadliest military conflict in American history, resulting in the deaths of approximately 618,000 soldiers as well as many civilians.^[102]
Following the Union victory in 1865, three amendments to the U.S. Constitution brought about the prohibition of slavery, gave U.S. citizenship to the nearly four million African Americans who had been slaves,^[103] and promised them voting rights. The war and its resolution led to a substantial increase in federal power^[104]aimed at reintegrating and rebuilding the Southern states while ensuring the rights of the newly freed slaves.^[105] Following the Reconstruction Era, throughout the South Jim Crow laws soon effectively disenfranchised most blacks and some poor whites. Over the subsequent decades, in both the North and the South blacks and some whites faced systemic discrimination, including racial segregation and occasional vigilante violence, sparking national movements against these abuses.^[105]