After graduating 66th in a class of 74 from West Point, Haskell initially served in the cavalry. He graduated from the Army Staff College in 1905 and then served from 1906 to 1907 in the Philippine–American War fighting against the Moro tribesmen. From November 1907 to November 1911, Haskell served in the Signal Corps. Promoted to captain effective 1 July 1916, he was assigned as temporary colonel of the 69th Infantry Regiment of the New York National Guard from 5 August 1916 to 9 March 1917 on the Mexican border. Haskell then served in the field artillery with the American Expeditionary Forces (AEF) during World War I, rising again to temporary colonel by 23 December 1918 and earning the Army Distinguished Service Medal.[1][2][3]
Reverting to captain in April 1920, Haskell was quickly promoted to major in July and lieutenant colonel in September.[2] He was assigned to the American Relief Administration in 1919 and served as its director in Soviet Russia from 1921 to 1923.[1] He retired from the army on 27 January 1926.[4]
On 19 May 1926, Haskell was appointed major general and commander of the New York National Guard by New York Governor Alfred E. Smith.[3][4] He received federal recognition as a major general on 28 May 1934.[5] Haskell served as commander of the New York National Guard for fourteen years until October 1940, when he was appointed commanding general of the 27th Infantry Division in preparation for American entry into World War II. He relinquished command of the division in October 1941 and retired from military service in April 1942. In appreciation for his service, the state of New York promoted him to lieutenant general.[1][3]