December 29th 1890-Miniconjou Chief Big Foot
Today was not the deadliest massacre in US history. Just off the top of my head here are two :
Sand Creek November 29, 1864
Colonel Chivington and his 800 troops of the First Colorado Cavalry, Third Colorado Cavalry and a company of First New Mexico Volunteers marched to their campsite in order to attack the Indians. On the morning of November 29, 1864, the army attacked the village and massacred most of its inhabitants. Chivington proclaimed before the attack "Kill and scalp all, big and little; nits make lice." Only 9 or 10 soldiers were killed and three dozen wounded. Between 150 and 184 Cheyennes were reported dead, and some were reportedly mutilated, and most were women, children, and elderly men. Chivington and his men later displayed scalp and other body parts, including human fetuses and genitalia in the Apollo Theater and saloons in Denver.
Wiki Page for Sand Creek
Wounded Knee December 29 1890
By the time it was over, 25 troopers and 300 Lakota Sioux lay dead, including men, women, and children. Many of the dead are believed to have been the victims of "friendly fire" as the shooting took place at point blank range in chaotic conditions, and most of the Lakota had previously been unarmed. Around 150 Lakota are believed to have fled the chaos, of which many likely died from exposure.
Wiki Page for Wounded Knee