American Civil War (1861-1865)

The American Civil War killed 620,000 to 750,000 soldiers in four years—more American deaths than all other U.S. wars combined. The industrial North's victory enforced a federal ban on slavery and unified the economy, propelling the U.S. to global superpower status by 1900. This conflict's 2.5% population loss far surpasses #1's U.S. impact proportionally, and the 13th Amendment's ratification in 1865 directly ended 250 years of legal slavery.