The commander I think you are talking about is Ulysses S. Grant
Abraham Lincoln as commander in chief, sent union troops to the South.
Ulysses S. Grant
McDowell
Winfield Scott Henry Halleck Ulysses S. Grant
General Ambrose Burnside
General George G. Meade
General George G. Meade
There were several Union generals in different theaters of war. Scott, Hallack, McClellan, Pope, Burnside, Hooker, Meade, and Grant all served as overall commander of the Union forces as various times.
The Union commander was Ulysses S. Grant
Alexander
The Union commander was Brigadier General Samuel Curtis, the Confederate Comamnder was Major General Earl Van Dorn.
When the union troops were in an area they often had the freed slaves come to them for protection and to help. At one point slaves came to one unit and the confederate officer demanded the return of the slaves from the union commander. The union commander told the Confederate that the slaves were contraband. By declaring them contraband he didn't have to return the slaves. After that event they called themselves "contraband."