American Civil War (1861–1865) was a major war between the United States ("Union") and eleven Southern states ("Confederacy"), which declared that they had a right to secession and formed the Confederate States of America, led by President Jefferson Davis. The Union included free states and border states and was led by President Abraham Lincoln and the Republican Party. Although the border states were under Union control, they supplied the South with tens-of-thousands of troops.

The South strongly believed in States' Rights (Bill of Rights and the 10th Amendment) according to the United States Constitution and believed that it entitled them to a right of secession. While the Republicans rejected any right of Southern secession, they also opposed the expansion of slavery into territories owned by the United States (see causes and origins of the American Civil War). Soldiers'  motives for fighting in the conflict also varied.

Fighting commenced on April 12, 1861, when Confederate forces attacked a United States (Federal) military installation at Fort Sumter in South Carolina, the first state to secede. South Carolina, however, claimed that Fort Sumter was legally within its (territorial) waters.

During the American Civil War, the North generally named a battle after the closest river, stream or creek, and the South tended to name battles after towns or railroad junctions. Hence the Confederate name Manassas after Manassas Junction, and the Union name Bull Run for the stream Bull Run.

During the first year of the Civil War, the Union assumed control of the border states and established a naval blockade as both sides raised large armies. In 1862, major bloody battles, such as Shiloh and Antietam, were fought causing massive casualties unprecedented in U.S. military history. In September 1862, Lincoln's Emancipation Proclamation made the freeing of slaves in the South a war goal, despite opposition from Northern Copperheads who tolerated secession and slavery. Emancipation reduced the likelihood of intervention from Britain and France on behalf of the Confederacy. In addition, the goal also allowed the Union to recruit African Americans for reinforcements, a resource that the Confederacy did not exploit until it was too late. The border states and War Democrats initially opposed emancipation, but gradually accepted it as part of total war needed to save the Union.

American Civil War Map

European immigrants joined the Union Army in large numbers too. 23.4% of all Union soldiers were German-Americans; about 216,000 were born in Germany. In the East, Confederate General Robert E. Lee assumed command of the Army of Northern Virginia and experienced a series of victories against the Army of the Potomac. However, Lee's best general, Thomas Jonathan "Stonewall" Jackson, was killed at the Battle of Chancellorsville in May 1863. Lee's invasion of the North was repulsed at the Battle of Gettysburg in Pennsylvania in July 1863; Lee, however, managed an orderly retreat to Virginia. The Union Navy captured the port of New Orleans in 1862, and Ulysses S. Grant seized control of the Mississippi River by capturing Vicksburg, Mississippi, in July 1863, thus splitting the Confederacy (see: Anaconda Plan: The United States Naval Plan of Divide and Conquer and Turning Points of the American Civil War).