The American Civil War (1861–1865) was a major war between the United States (the "Union") and eleven Southern slave states that declared their secession and formed the Confederate States of America, led by President Jefferson Davis. The Union, led by President Abraham Lincoln and the Republican Party, opposed the expansion of slavery and rejected any right of secession.
During the first year, the Union asserted control of the border states and established a naval blockade as both sides raised large armies. In 1862 large, bloody battles began, causing massive casualties as a result of new weapons and old battlefield tactics.
In the West, 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.
Grant fought a number of bloody battles with Lee in Virginia in the summer of 1864. Lee won most of the battles in a tactical sense but on the whole lost strategically, as he could not replace his casualties and was forced to retreat into trenches around his capital, Richmond, Virginia.
In 1865, the Confederacy collapsed after Lee surrendered to Grant at Appomattox Court House.
The war produced about 970,000 casualties (3% of the population), including approximately 620,000 soldier deaths—two-thirds by disease. The causes of the war, the reasons for its outcome, and even the name of the war itself are subjects of lingering controversy even today.