Penn graduated from West Point in 1886, ranked 69th of 77, and was commissioned as a
second lieutenant in the
13th Infantry. Penn formed a lifelong friendship with classmate
John J. Pershing, and after fellow Army officer
Richard B. Paddock married Pershing's sister Grace, Pershing, Paddock and Penn socialized extensively while assigned to
Fort Stanton,
New Mexico. They hired a cook to grow vegetables and prepare meals for all three, and went on extensive hunting and fishing trips, which led to them being dubbed “The Three Green Ps”. While in New Mexico, Penn participated in an 1886 expedition against members of the
Jicarilla Apache band, who were resisting attempts to be settled on a reservation. In 1887, he was involved in an expedition against members of the
Mescalero tribe who had left their reservation. During the early years of his career, Penn served throughout the Western United States, including postings to
Oklahoma,
Wyoming and
Idaho. He graduated from the Infantry and Cavalry School at Fort Leavenworth in 1891 (now the
Command and General Staff College), and his thesis on mounted infantry was published in the
Journal of the Military Service Institution of the United States. After graduation, he was assigned to the
2nd Infantry and in addition to his regular duties he was assigned to
Omaha High School as an instructor in military tactics for the school's corps of cadets. In 1894, he was in
Butte, Montana, as part of the Army's response to labor unrest during the
Pullman Strike. In 1895, he took part
in an expedition against the
Bannock Indians, which was organized after false reports of an imminent uprising at the Bannock reservation in Wyoming. ==Spanish–American War==