slave societies. The United States became ever more polarized over the issue of slavery, split into slave and free states. Driven by labor demands from new cotton plantations in the Deep South, the Upper South sold more than a million slaves who were taken to the Deep South. The total slave population in the South eventually reached four million. As the United States expanded, the Southern states attempted to extend slavery into the new western territories to allow proslavery forces to maintain their power in the country. The new territories acquired by the Louisiana Purchase and the Mexican Cession were the subject of major political crises and compromises. By 1850, the newly rich, cotton-growing South was threatening to secede from the Union, and tensions continued to rise. Bloody fighting broke out over slavery in the Kansas Territory. Slavery was defended in the South as a "positive good", and the largest religious denominations split over the slavery issue into regional organizations of the North and South. When Abraham Lincoln won the 1860 election on a platform of halting the expansion of slavery, seven slave states seceded to form the Confederacy. Shortly afterward, on April 12, 1861, the Civil War began when Confederate forces attacked the U.S. Army's Fort Sumter in Charleston, South Carolina. Four additional slave states then joined the Confederacy after Lincoln, on April 15, called forth in response "the militia of the several States of the Union, to the aggregate number of seventy-five thousand, in order to suppress" the rebellion. During the war some jurisdictions abolished slavery and, due to Union measures such as the Confiscation Acts and the Emancipation Proclamation, the war effectively ended slavery in most places. After the Union victory, the Thirteenth Amendment to the United States Constitution was ratified on December 6, 1865, prohibiting "slavery [and] involuntary servitude, except as a punishment for crime." Origins First enslavements In 1508, Juan Ponce de León established the Spanish settlement in Puerto Rico, which used the native Taínos for labor. The Taínos were largely exterminated by war, overwork and diseases brought by the Spanish. In 1513, to supplement the dwindling Taíno population, the first enslaved African people were imported to Puerto Rico. The abolition of Indian slavery in 1542 with the New Laws increased the demand for African slaves. A century and a half later, the British conducted enslaving raids in what is now Georgia, Tennessee, North Carolina,