What happened at the Battle of the Little Bighorn?

The full answer to this question would encompass enough information to fill a book, and has.

To put it as simply as possible; US Army General Custer under estimated the size, strength and tactics of a group of Native American's (Sioux & Cheyenne) and the ensuing battle resulted in a complete victory by the Native Americans.

The Battle of Little Big Horn took place in Montana on June 25, 1876. Under the command of General Sheridan was the 7th Calvary headed by Lt. Col. George Custer. US President Grant had ordered all Native American tribes in the Black Hills area of Montana onto reservations by January 31, 1876. If they declined the army would force compliance. The Native American Tribes refused to comply with what they considered to be an unfair situation. As part of this compliance, Custer was conducting operations in the valley of the Little Big Horn.

Custer made the error of splitting his forces and the troops under his direct command fell under attack of Sitting Bull and his warriors. The numbers of fighters on each side has been disputed but for arguments sake Custer had 200 plus troopers and Sitting Bull had about 4,000 warriors (this is one estimation ). Within about 45 minutes, Custer's troops were wiped out to the man. A number of military historians do not consider this to be a battle because of the length of time and the numbers of cavalry troops involved. They have used the Word "confrontation" rather than battle.