In 1861, Abraham Lincoln is elected as president. The South, already fed up with the North, is appalled by the election of a supposedly anti-slavery president.
So appalled, in fact, that they determine complete secession to be justifiable. By the end of 1861, 11 southern states had seceded.
Fort Sumter, SC-April 12, 1861
Southwestern Tennessee- April 6, 1862
THE CONFEDERATES ARE COMING!
The U.S. government fails to recognize their succession as legitimate, and refuses to turn over government property to the Southerners. This ends in a 3 1/2 day siege. This further enrages the Southerners.
Anticipating southern aggression because of the attack on Fort Sumter, President Lincoln orders union troops to march towards Richmond. They are intercepted, however, and the ensuing battle (First Battle of Bull Run) ends disastrously for union troops.
Just north of Manassas-July 21, 1861
Bull run was only the beginning of a series of batterings which were eventually responsible for Lincoln's hiring of Ulysses S. Grant.
Several battles later...
The hiring of grant subsequently led to the Union's first major victory at the Battle of Shiloh- one of the major turning points in the war.