Which US military force was all but annihilated during the battle of "Greasy Grass?"

The Battle of the Greasy Grass as it was known to the Lakota Sioux, is actually referred to as the The Battle of the Little Bighorn by the military, and commonly called Custer's Last Stand. It was an armed engagement between combined forces of the Lakota, Northern Cheyenne, and Arapaho tribes, against the 7th Cavalry Regiment of the United States Army.

The battle, which occurred June 25–26, 1876 near the Little Bighorn River in eastern Montana Territory, was the most prominent action of the Great Sioux War of 1876.

The fight was an overwhelming victory for the Lakota, Northern Cheyenne, and Arapaho, led by several major war leaders, including Crazy Horse and Chief Gall, and inspired by the visions of Sitting Bull.

The U.S. 7th Cavalry, including the Custer Battalion, a force of 700 men led by George Armstrong Custer, which attacked the Indian encampment at area known to the Indians as the Greasy Grass, suffered a major defeat and was all but annihilated.