Fort Sumter: April 1861, a federal fort in the harbor of Charleston, South Carolina. Confederate forces staged a 24-hour bombardment against it and by attacking federal property, had committed an act of open rebellion. To uphold the Constitution, President Lincoln believed he had no choice but to call fro troops to respond against the Confederacy. As a direct result, the Civil War began.
Antietam: September 1862, Robert E. Lee marched his forces to Antietam Creek, Maryland, where he fought the first major battle in the north. The outcome of this battle was Lincoln using the Emancipation Proclamation.
Gettsyburg: July 1863, Robert E. Lee hoped that an invasion of Union territory would significantly weaken Northern support for the war effort. He wanted to get support from Great Britain or France. It was the deadliest battle of the American Civil War.
Vicksburg: May-July 1863, Ulysses S. Grant laid siege to Vicksburg, Mississippi, because the army that controlled its high ground over a bend in the Mississippi River would control traffic on the whole river. Union Victory.
Atlanta: July-September 1864, William Sherman besieged Atlanta, Georgia. His goal was to disrupt the Confederacy's capacity to resupply its troops throughout the South. He burned buildings and tore up railways. This decided who the winner was.