The Civil War began in April 1861. Ulysses felt it was his duty to re-enter the military having been educated at the federal government's expense at West Point. He soon rose to fame following battles at Forts Henry and Donelson, along with the new nickname, "Unconditional Surrender" since those were the terms he gave to the Confederate forces at Donelson.
Other battles, especially Vicksburg, brought Grant to the attention of President Abraham Lincoln, who named him general of all Union armies in March 1864. Grant supported Lincoln's war aims of union and freedom for the nearly 4 million African American enslaved individuals.
Following Lincoln's enactment of the Emancipation Proclamation, Grant made sure that the enslaved men, women, and children who escaped to Union lines were protected and cared for. After being promoted, Grant worked diligently to bring an end to the war, accepting Confederal General Robert E. Lee's surrender in April 1865, effectively ending the Civil War.
U.S. Grant Civil War bio provided by the National Park Service