In 1903, Truesdell took part in the competitive examination process for appointment as an officer. He passed, and in June 1904 he received his commission as a
second lieutenant in the
5th Infantry Regiment. Truesdell joined his regiment at
Plattsburgh Barracks,
New York and his initial assignments included escorting military prisoners from the
Castle Williams penitentiary on
Governors Island, New York to the
United States Disciplinary Barracks at
Fort Leavenworth,
Kansas. In August 1907, Truesdell was assigned as quartermaster and commissary officer of the 5th Infantry's 2nd Battalion and posted to
Sagua La Grande during the
Second Occupation of Cuba. In March 1911, Truesdell was promoted to
first lieutenant in the
15th Infantry Regiment. In March 1912, Truesdell was detailed to the
Signal Corps. After beginning the Signal Officers Course at Fort Leavenworth's Army Service Schools, he completed the course with
Field Company A, Signal Corps, at
Fort Omaha,
Nebraska. On May 29, 1912, Truesdell was one of several officers taking part in a
hot air balloon flight to learn how balloons facilitated radio communications. What was supposed to be a short familiarization flight over Fort Omaha became the subject of nationwide headlines on May 29 and 30 when the pilot lost control and the balloon was no longer visible to observers on the ground. In May 1913, Truesdell relinquished command and was assigned as assistant to the chief signal officer on the staff of the army's
Eastern Department, which was based at
Fort Jay on Governors Island. In February 1914, was assigned to duty with Field Company I, Signal Corps, at
Fort Bliss,
Texas. In April 1914, he was reassigned to
Fort Sam Houston, Texas for duty with Field Company H, Signal Corps. In February 1915, Truesdell was assigned to the
25th Infantry Regiment with duty at
Texas City, Texas. In July 1915, he returned to Plattsburgh Barracks as inspector of training for an army instruction encampment. In December 1915, Truesdell rejoined the 25th Infantry at
Schofield Barracks,
Hawaii. He was promoted to
captain in November 1916 and assigned as commander of the regiment's Company I. ==World War I==