Sheridan was commissioned as a brevet second lieutenant and was assigned to the 1st U.S. Infantry Regiment at Fort Duncan, Texas, then to the 4th U.S. Infantry Regiment at Fort Reading, California. Most of his service with the 4th Infantry was in the Pacific Northwest, starting with a topographical survey mission to the Willamette Valley in 1855, during which he became involved with the Yakima War and Rogue River Wars, gaining experience in leading small combat teams, being wounded (a bullet grazed his nose on March 28, 1857, at Middle Cascade, Oregon Territory), and some of the diplomatic skills needed for negotiating with Indian tribes. He lived with a mistress during part of his tour of duty, an Indian Rogue River woman and daughter of Takelma Chief Harney, named Frances by her white friends. He was promoted to first lieutenant in March 1861, just before the Civil War, and to captain in May, immediately after Fort Sumter.