 Episcopal Bishop Henry Benjamin Whipple pleaded to President Abraham Lincoln for clemency , and the death sentences of all but 39 men were reduced to prison terms . On December 26 , 1862 , 38 men were hanged by the U.S. Army at Mankato — the largest mass execution in the United States . Many of the remaining Dakota Native Americans , including non @-@ combatants , were confined in a prison camp at Pike Island over the winter of 1862 – 1863 , where more than 300 died of disease . Survivors were later exiled to the Crow Creek Reservation , then later to a reservation near Niobrara , Nebraska . 
