Present-day Powder River County was probably first entered by Europeans in the early 1800s in the form of French fur trappers. In the summer of 1865, the United States government sent over 2,000 U.S. soldiers into the Powder River country to combat Native Americans from the Cheyenne people, Lakota people, and Arapaho people tribes. From September 1-15, 1865, the Powder River Expedition (1865) battled Native Americans in the Powder River Battles (1865) near the future site of Broadus. On March 17, 1876, the Battle of Powder River occurred in the south-central part of the county, about  southwest of Broadus. In June 1876 six companies of the 7th Cavalry Regiment (United States) led by Major Marcus Reno marched along the Powder River nearly to the mouth of the Little Powder before turning west toward the Battle of Little Bighorn. On November 1, 1878, Powderville, Montana became the first establishment in the county, as the Powder River Telegraph Station, on the Fort Keogh to Deadwood, South Dakota telegraph line. On April 5, 1879, the Mizpah Creek Incidents began near the Powderville telegraph station in the extreme northern part of the county. In the early 1880s, a large area of Southeastern Montana became Custer County, Montana, including the present area encompassed by Powder River County. In February 1900, the Broadus post office was established, named after the Broaddus family. In October 1918, publication of a newspaper, the Broadus Independent (now called the Powder River Examiner), began in Broadus, and is still published today. In 1919, Powder River County, Montana was formed from southern Custer County, and in an election in 1920, Broadus was chosen as the county seat of the newly established Powder River County.
Question: How many days did the Powder River Expedition battle Native Americans in the Powder River Battles?
Answer: 15