The Battle of Ash Hollow, also known as the Battle of Blue Water Creek or the Harney Massacre, was an engagement of the First Sioux War, and fought on September 2 and 3, 1855 between United States Army soldiers under Brig. Gen. William S. Harney and a band of the Brulé Lakota along the Platte River in present-day Garden County, Nebraska. In the 20th century, the town of Lewellen, Nebraska, was developed here as a railroad stop.

The Sioux Wars were a series of conflicts between the United States and various subgroups of the Sioux people which occurred in the later half of the 19th century. The earliest conflict came in 1854 when a fight broke out at Fort Laramie in Wyoming, when Sioux warriors killed 31 American soldiers in the Grattan Massacre, and the final came in 1890 during the Ghost Dance War.