The American Civil War begins
Fort Sumter in Charleston, South Carolina, was under threat from the Confederacy even before Lincoln took office in March 1861.
In response to Lincoln's command to send a ship to resupply Sumter, Confederate artillery opened fire on April 12, starting the Civil War. Fort Sumter was captured by the Confederates under the direction of Pierre G. T. Beauregard when its commander, Major Robert Anderson, surrendered after less than two days of bombardment. Virginia, Arkansas, North Carolina, and Tennessee were the next four states to join the Confederacy after Fort Sumter was captured. Missouri, Kentucky, and Maryland did not separate because of their proximity to the Confederacy, but their residents had a strong pro-Confederate stance.
The Union's overwhelming population
Railroad construction advantages over its Confederate counterparts
However, despite the Union's overwhelming population, manufacturing (including arms production), and railroad construction advantages over its Confederate counterparts, the Civil War was fought by two sides with strong military traditions and some of the country's best soldiers and commanders on both sides. It was important to them that their long-held traditions and institutions, like as slavery, were preserved.