The American Civil War was the costliest war for the U.S. in terms of casualties. After most slave states, located in the southern U.S., formed the Confederate States, C.S. troops led by former U.S. Army officers, mobilized a very large fraction of Southern white manpower. Forces of the United States (the "Union" or "the North") formed the Union Army consisting of a small body of regular army units and a large body of volunteer units raised from every state, north and south, except South Carolina.[citation needed]