He had an impressive early infantry career

It is a fact that he had an impressive early infantry career. Marshall went on to serve as Commandant of Students at Danville Military Institute in Danville, Virginia, after graduating from VMI. He applied for a commission in the United States Army, which had been significantly expanded to deal with the Spanish-American War and the Philippine-American War. Marshall was elected and used endorsements from both of Pennsylvania's U.S. Senators to strengthen his application. VMI Superintendent Scott Shipp also endorsed Marshall's application, comparing him favorably to other VMI alumni serving in the Army, saying Marshall was fully the equal of the best. In February 1902, he was commissioned as a second lieutenant of infantry.

Prior to World War I, Marshall served as an infantry platoon leader and company commander throughout the Philippine-American War and other guerrilla movements in the United States and the Philippines. He received training in modern warfare, including a tour as both a student and a teacher at Fort Leavenworth, Kansas, from 1906 to 1910. He graduated first in his Army Staff College class in 1908 and was the Honor Graduate of his Infantry-Cavalry School Course in 1907.

Marshall returned to the Presidio of San Francisco in 1916 after serving as aide-de-camp to the commander of the Western Department, former Army chief of staff Major General J. Franklin Bell. Marshall and Bell traveled to Governors Island, New York after the United States declared war on Germany in April 1917, when Bell was posted as head of the Department of the East. Marshall was soon sent to help oversee the mobilization of the 1st Division for service in France.