In response to the 1860 election of Abraham Lincoln, a fierce opponent of slavery, as President of the United States, the slave-holding states of South Carolina, Alabama, Mississippi, Georgia, Florida, Louisiana, and Texas seceded from the Union and formed their own nation, the Confederate States. They demanded the unconditional withdrawal of all US military forces in their own territory, including Fort Sumter in Charleston, South Carolina--despite the fact that Fort Sumter was owned by the Federal government, not South Carolina or any other state, loyal or Confederate. Rebel forces fired on Fort Sumter on April 12, 1861, beginning the war.
Lincoln called on the loyal states to raise a volunteer army to suppress the rebellion. Most states complied with the request, but the slave-holding states of Virginia, Arkansas, North Carolina, and Tennessee were offended by the idea of "betraying" their fellow Southern states and instead threw in their lot with the Confederacy.
The first major action of the war came in July 1861 when undertrained Union and Confederate forces met at Manassas, Virginia, by Bull Run. Both sides expected an easy victory, but the undisciplined Union troops were routed and fell all the way back to Washington, DC and the Confederate forces, in little better shape, were unable to press their advantage.
Despite small campaigns in Missouri, the newly formed West Virginia and elsewhere, there was no more major action that year as the two sides' armies organized within their own territory. The following spring, the massive Union Army of the Potomac, the largest army in the history of the US up to that point, was dispatched to the Virginia Peninsula to move against Richmond. Unfortunately, it was commanded by the extremely timid General George McClellan. McClellan was extremely hesitant to press his many advantages in the offensive campaign, falsely believing he was outnumbered by the Confederate Army of Northern Virginia despite overwhelming evidence to the contrary in his Intelligence reports. After halting the advance when the original ANV commander John Magruder built false artillery out of painted logs, and being similarly held up by subsequent ANV commander Joseph Johnson, McClellan was repulsed from the Peninsula altogether when he first met Confederate General Robert E. Lee.
The following summer, Lee defeated the Union Army of Virginia under John Pope at a second battle of Bull Run.
Union forces under General Ulysses S. Grant had won several important victories farther west, including the Battle of Shiloh, at which Confederate General Albert Sidney Johnston was killed in action. The Union was also winning the naval war, with the USS Monitor defeating the CSS Virginia in the world's first battle between ironclad warships, and the US Navy establishing a blockade of Southern ports which the CS Navy was helpless to break. However, the East was the war's primary front, and in the fall of 1862 Confederate fortunes were riding extremely high, with Britain and France considering granting the CS diplomatic recognition. That fall, Lee launched an ambitious invasion of Maryland and Pennsylvania. In the West, Braxton Bragg led the Army of Tennessee on a simultaneous invasion of the state of Kentucky. McClellan was, as usual, extremely slow to respond to Lee's invasion, and Union intelligence failed to realize that the Army of Northern Virginia had adopted a high-risk marching order in which each division of James Longstreet's and Thomas Jackson's two corps were all marching alone. (A courier riding from Lee's headquarters to Daniel Harvey Hill's dropped a case of three cigars containing Special Orders 191 outside of Frederick, Maryland but immediately recovered them when two Confederate infantrymen saw him drop it. Had he failed to realize he'd dropped it, it would most likely have come into possession of Union forces who soon took up possession of the ground.) McClellan made the typically foolish decision to offer Lee battle at Camp Hill, Pennsylvania, where the Army of the Potomac was destroyed on October 1, 1862.
Lee advanced unopposed on the city of Philadelphia and took possession of it, while Bragg, after defeating U.S. forces at Corinth, Kentucky, conquered the state and moved it into the Confederate States by force. Meanwhile, Britain and France extended diplomatic recognition to the Confederate States, and the war formally ended on November 4, 1862, when British Ambassador Lord Lyons visited President Lincoln in the Executive Mansion to deliver the veiled threat of the Royal Navy crossing the Atlantic to break the Union blockade of Southern ports. Lincoln was forced to grant diplomatic recognition to the Confederate States and accept British Foreign Secretary John Russell's offer of mediation between the two states; the decision would forever ruin Lincoln's reputation as a good President in US history. (Fifty-five years later, President Theodore Roosevelt would contend that Lincoln had done so under durress, and that subsequent US governments were therefore not bound to honor his agreement with the Rebels.)
Despite Ambassador Lyons' prediction that "in time, the United States and the Confederate States, still having between them a common language and much common history, shall take their full and rightful places in the world, a pair of sturdy brothers," the two nations remained bitter rivals and enemies for the next eighty-one years and counting; millions of Americans and Confederates born long after the war despised each other with an intensity at least equal to that of their forebears. The two states would fight three more wars: the Second Mexican War (1881-82); the Great War (1914-17); and the Second Great War (1941-44).