During his junior high school years, Fields entered the Maryland National Guard and served continuously with that organization until January 1932.

He struggled to find a job during the Great Depression, but decided to enlist in the Marine Corps in January 1932, hoping to obtain a commission.

The class graduated future Marine Corps Commandants Leonard F. Chapman Jr. and Robert E. Cushman Jr., lieutenant generals Herman Nickerson Jr., William J.

Fields was subsequently attached to the Marine detachment aboard the newly commissioned cruiser USS Quincy and took part in the evacuation of the United States nationals during the Spanish Civil War.

After graduating, he was promoted to first lieutenant in July 1938, and was appointed aide de camp to Richard P. Williams, the commanding general of the Fleet Marine Force.

He served in this capacity until November 1941, when he was promoted to the rank of captain and sent to the Battery Commander course at Field Artillery School at Fort Sill, Oklahoma.

[2] Upon the completing the course in March 1942, Fields was transferred to the 3rd Battalion, 11th Marine Artillery Regiment at Camp Lejeune, North Carolina.

He took part in the planning and coordination of Peleliu and Ngesebus operation between September and November 1944 and was decorated with the Legion of Merit with Combat "V" for his service.

Closely collaborating with staff members in the various Naval Attack Forces, he prepared operational orders for the embarkation and landing, effectively coordinating them with the inland scheme of maneuver.

Preparing plans and orders for the combined elements of the United States Armed Forces, he contributed materially to the seizure of the hazardous terrain dominating the airdromes on each island, thus insuring their complete security.

Fields was promoted to the rank of colonel in May 1950 and joined Supreme Allied Command, Atlantic, as assistant chief of staff for personnel and administration under Admiral Lynde D. McCormick in January 1951.

[2][3] Fields was subsequently attached to the Headquarters Marine Corps in Washington, D.C., and appointed head of the Plans Branch within Division of Operations.

[2] Fields returned to the United States in April 1954 and following the brief leave, he was sent to Europe to the Supreme Headquarters, Allied Powers in Paris, as Chief of Plans Section under General Alfred Gruenther.

His service in Europe ended in September 1956, when he was transferred to the headquarters of Fleet Marine Force, Atlantic at Norfolk, Virginia and appointed assistant chief of staff, G-3 under Lieutenant General Alfred H. Noble.

[2] The several units of 1st Marine Division were deployed to South Vietnam during the rest of 1965 and participated in the amphibious operations as a part of Seventh Fleet landing force.

Fields planned and directed Operation Hot Springs in April 1966, during which his Marines killed over 150 enemy soldiers and captured a large number of weapons.

Responsible for the education of all Marines, with particular emphasis on the professional training of Marine Corps officers in all aspects of amphibious operations, Lieutenant General Fields skillfully implemented a comprehensive, progressive program of basic, intermediate, and advanced instruction, contributing significantly to the high level of leadership and readiness of the Corps by providing highly competent commanders and staff officers for the air-ground forces.

During this period, more than 26,000 Marines, as well as officers of the United States Army, Navy, Air Force, and over forty foreign countries, successfully advanced their military education and professional competence in courses of instruction that visibly reflected Lieutenant General Fields' own personal character, experience, and refined capacity for planning, organization, and management.