After the Civil War began in April 1861, Grant became a colonel of the 21st Illinois Volunteers. Later that summer, President Abraham Lincoln (1809-1865) made Grant a brigadier general. Grant’s first major victory came in February 1862, when his troops captured Fort Donelson in Tennessee. When the Confederate general in charge of the fort asked about terms of surrender, Grant famously replied, “No terms except an unconditional and immediate surrender can be accepted.”
Grant’s first major victory came in February [16th] 1862, when his troops captured Fort Donelson in Tennessee. When the Confederate general in charge of the fort [Simon Bolivar Buckner] asked about terms of surrender, Grant famously replied, “No terms except an unconditional and immediate surrender can be accepted.”http://www.history.com/topics/us-presidents/ulysses-s-grant
"Grant gave the Union Army its first major victory of the American Civil War by capturing Fort Henry, Tennessee, on February 6, 1862. . ."
I knew this already, but thought I should cite a source: http://www.thelatinlibrary.com/chron/civilwarnotes/grant.html
Belmont, Missouri was his first battle, not a real victory. Fort Henry was his first followed by Fort Donelson, which was the battle that made him, his first major victory. Shiloh of course was his biggest victory early on.
I have finally lost my mind. I have found too many hits saying Fort Donelson was Grants first major win. Therefore I am going back to my original answer of Fort Donelson.
So sorry Hoosier for. Please forgive my mental incompetence.