At the start of the War of Independence (1776–83), slavery was legal throughout the thirteen colonies. In its aftermath, however, the northern states introduced legislation for abolition: in some abolition was immediate, in others gradual. The 1787 North West Ordinance prohibited slavery in the territories that would become Ohio, Indiana, Illinois, Wisconsin and Michigan, extending to Iowa and Minnesota upon their incorporation in 1820. An 1808 Act of Congress banned the import of slaves from overseas. An increasingly stark contrast was emerging between North and South, as dependence on mass slavery intensified in the southern states with the explosive growth in plantation farming of cotton. In 1860 Abraham Lincoln won the presidency on a platform of preventing expansion of slavery, not abolition; considering this inflammatory, the southern states seceded and civil war ensued. During the war, Lincoln made his Emancipation Declaration (1863), which became law by the Thirteenth Amendment in 1865.