Belknap was the commander of the destroyer USS Ammen (DD-35) in 1913 and attended the Naval War College from 1915 to 1916.

[1] Due to a shortage of experienced officers, he received a temporary promotion to commander on January 1, 1918.

On January 9, 1918, shortly after his promotion to commander, Belknap was appointed director of the newly formed Naval Overseas Transportation Service (NOTS).

In this position, Belknap was responsible for Navy cargo ships and all auxiliary vessels not assigned to the fleet or the various naval districts.

He was one of a very few individuals to receive both the Navy Cross and the Army Distinguished Service Medal.