Lincoln took office following the 1860 presidential election, in which he won a plurality of the popular vote in a four-candidate field. Almost all of Lincoln's votes came from the Northern United States, as the Republicans held little appeal to voters in the Southern United States. A former Whig, Lincoln ran on a political platform opposed to the expansion of slavery in the territories. His election served as the immediate impetus for the outbreak of the American Civil War. After being sworn in as president, Lincoln refused to accept any resolution that would result in Southern secession from the Union. The Civil War began weeks into Lincoln's presidency with the Confederate attack on Fort Sumter, a federal installation located within the boundaries of the Confederacy.