Simpson's first assignment was with the 2nd Battalion,
6th Infantry Regiment, which was stationed at
Fort Lincoln,
North Dakota. He acquired the nickname "Big Simp" because there was another officer in the regiment with the same surname. Because Simpson was over tall, he became "Big Simp" and the other officer became "Little Simp". Soon after he joined in the regiment on 11 September 1909, it received orders to deploy to the Philippines. He embarked from
San Francisco on 5 January 1910. He went to the island of
Mindanao in the Philippines and participated in suppression of the
Moro Rebellion. He returned to the United States with his regiment, arriving at the
Presidio of San Francisco on 10 July 1912. The regiment moved to
El Paso, Texas, between 24 April and 1 May 1914. Promoted to
first lieutenant on 1 July 1916, he commanded Companies C and K in the
Pancho Villa Expedition in 1916. On 24 February 1917, he became
aide-de-camp to
Brigadier General George Bell Jr., the commander of the El Paso Military District. Simpson was promoted to
captain on 15 May 1917, a month after the
American entry into World War I. He followed Bell on a tour of inspection of the
British and
French forces on the
Western Front in September and October 1917. They then returned to
Camp Logan, Texas, where the
33rd Division was activated, with Bell, now a
major general, as its first commander. The 33rd Division arrived in France in May 1918 and Simpson became its Assistant Chief of Staff (G-3), the
staff member responsible for
operations. He was promoted to
major on 7 June 1918, and attended the
American Expeditionary Forces (AEF) Army General Staff College from 15 June to 30 August. He returned to the 33rd Division as assistant to its G-2 (the staff member responsible for
intelligence) on 1 September, then became assistant to its G-3 on 15 September. He became G-3 again on 4 October, and participated in the
Meuse–Argonne offensive. He was promoted to
lieutenant colonel on 5 November, and became the division's
chief of staff on 17 November, soon after
World War I ended on
11 November 1918. For his services during the war he was awarded the
Army Distinguished Service Medal and
Silver Citation Star, and the French
Croix de guerre and
Legion of Honour in the grade of Chevalier. The citation for his Army DSM read: ==Between the wars==