A flag carried by Lt Col George Armstrong Custer and his 7th Cavalry troops into their last stand at the Battle of Little Bighorn has been sold at auction for $2.2m (£1.4m).
The guidon is the only one not captured or lost during the 1876 battle in the state of Montana.
The flag, previously valued at $5m, was bought by a private US collector in the auction at Sotheby's in New York.
The former owner, Detroit Institute of Arts, paid $54 for the flag in 1895.
"We'll be using the proceeds to strengthen our collection of Native American art, which has a rather nice irony to it I think," said Graham Beal, director of the Detroit museum.
Lt Col Custer and all his soldiers - more than 200 in number - were killed by thousands of Lakota Sioux and Northern Cheyenne warriors during their attempt to reclaim the Black Hills region from the Lakota as part of a US government campaign.
The flag was found beneath a dead American soldier following the Battle of Little Bighorn - or the Battle of Greasy Grass Creek, as the victors of the battle named it.
The flag was renamed Culbertson Guidon after Sgt Ferdinand Culbertson, a member of the burial party who recovered it from the field.
Sitting Bull and Crazy Horse were among the Lakota leaders who fought in the battle.