slaves upstream to markets or plantation destinations at Natchez and Memphis. Secession and the American Civil War (1860–1865) With its plantation economy, Louisiana was a state that generated wealth from the labor of and trade in enslaved Africans. It also had one of the largest free black populations in the United States, totaling 18,647 people in 1860. Most of the free blacks (or free people of color, as they were called in the French tradition) lived in the New Orleans region and southern part of the state. More than in other areas of the South, most of the free people of color were of mixed race. Many gens de couleur libre in New Orleans were middle class and educated; many were also property owners. In contrast, according to the 1860 census, 331,726 people were enslaved, nearly 47% of the state's total population of 708,002. Construction and elaboration of the levee system was critical to the state's ability to cultivate its commodity crops of cotton and sugar cane. Enslaved Africans built the first levees under planter direction. Later levees were expanded, heightened and added to mostly by Irish immigrant laborers, whom contractors hired when doing work for the state. As the 19th century progressed, the state had an interest in ensuring levee construction. By 1860, Louisiana had built of levees on the Mississippi River and another of levees on its outlets. These immense earthworks were built mostly by hand. They averaged six feet in height, and up to twenty feet in some areas. Enfranchised elite whites' strong economic interest in maintaining the slave system contributed to Louisiana's decision to secede from the Union in 1861. It followed other Southern states in seceding after the election of Abraham Lincoln as President of the United States. Louisiana's secession was announced on January 26, 1861, and it became part of the Confederate States of America. The state was quickly overtaken by Union troops in the American Civil War, a result of Union strategy to cut the Confederacy in two by seizing the Mississippi. Federal troops captured New Orleans on April 25, 1862. Due to a large enough portion of Louisiana's population being Southern Unionist and having Unionist leanings, (or compatible commercial interests), the Federal government took the unusual step of designating the areas of Louisiana under Federal control as a state within the Union, with its own elected representatives to the U.S. Congress. Late