Who was considered by President Grant and his advisors to be a "fool" for his slaughter at this battle?
President Grant was referring to George Armstrong Custer, and his defeat at the Battle of Little Big Horn. This battle is usually called Custer's Last Stand.
The general perception of this battle is that Custer rushed into an area without being prepared. However, the actual story is long and complicated. Custer had been a very successful leader for the Union forces during the Civil War, and had continued his success over the next ten years. He led the 7th Calvary in routing Native Americans from areas of the Plains. He was noted, both during the Civil War and during his fights with the Native Americans as being extremely good at using surprise attacks and in using the terrain to his advantage. While his defeat at Little Big Horn has variously been portrayed in movies and books as both courageous or stupid, the reality appears to be that some of his troops did not obey orders. Historians are still arguing about what really happened.