During World War I, he served with the 1st Engineers on the Western Front, and was awarded the Distinguished Service Cross and the French Croix de guerre.

During the Korean War, Farrell returned to active duty once more, serving with the Defense Production Administration, and then with the Atomic Energy Commission as its Assistant General Manager for Manufacturing.

[3] The 1st Engineers' main role was maintenance of the roads and construction of bridges in the 1st Division area, although detachments also employed Bangalore torpedoes to clear paths through barbed wire.

His citation read:for extraordinary heroism in action while serving with 1st Engineers, 1st Division, A.E.F., at Bois-de-Moncy, France, October 8–9, 1918.

On October 8 when ordered to take and hold Hill 269, which was strongly held by enemy forces, Major Farrell with great skill and with undaunted courage and determination led his battalion to the attack, seized and held this vital point despite the fact that he was attacked by greatly superior numbers on three sides and nearly surrounded by strong enemy forces who showed extraordinary determination to regain this highly important position.

His fearless leadership, utter disregard for his own safety, and complete devotion to duty raised the morale of his battalion to a high pitch and inspired them to acts of great endeavor.

[7]Farrell was also awarded the Croix de guerre with palm for his actions,[8] and the 1st Battalion received a citation from Major General Charles Summerall, the commander of V Corps.

[9] After the Armistice with Germany in November 1918, the 1st Engineers participated in the occupation of the Rhineland, with Farrell's 1st Battalion basing itself at Ebernhahn.

[16] Farrell and Groves worked out new, simplified procedures for centralized procurement that provided the flexibility needed to get projects done on time with the accountability that such enormous expenditures demanded.

In addition to this work, Farrell had to support Operation Matterhorn, the deployment of B-29 bombers to China and India, which involved the construction and expansion of a series of air bases.

The B-29s required runways that were almost twice the size of those for the older B-17s,[20] and he was forced to divert his resources to construct a 6-inch (150 mm) oil pipeline to the Matterhorn airfields.

[7] In December 1944, the Secretary of War, Henry L. Stimson, ordered Groves, now the Director of the Manhattan Project, to find a deputy.

"[25] "Site Y" was the code name for the remote Los Alamos County, New Mexico facilities that housed the main group of researchers and was responsible for final assembly of the bombs.

[31] In his report on the test to President Truman on 21 July 1945,[32] Farrell stated: The effects could well be called unprecedented, magnificent, beautiful, stupendous, and terrifying.

No man-made phenomenon of such tremendous power had ever occurred before...It lit every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined.

Seconds after the explosion came, the air blast pressed hard against the people watching, to be followed almost immediately by the strong, sustained, awesome roar which warned of doomsday and made us feel we puny things were blasphemous to dare tamper with the forces previously reserved for the Almighty.

"[40] After the bombing of Hiroshima on 6 August, Farrell, along with Generals Carl Spaatz, Nathan Twining, Barney Giles and James H. Davies, debriefed Parsons, the aircrews and the observers, and sent Groves a detailed report.

Farrell arrived in Hiroshima by air on 8 September as part of a group, equipped with portable geiger counters, that was headed by himself, and also included Brigadier General James B. Newman, Jr from the US Army Air Forces, Japanese Rear Admiral Masao Tsuzuki, who acted as a translator, and Colonel Stafford L. Warren, the head of the Manhattan District's Medical Section.

They were greatly impressed by both the damage done by the atomic bombs, and the extensive Japanese preparations for the Allied invasion that had been planned prior to the surrender.

[3] He was appointed chairman of the New York City Housing Authority by Mayor William O'Dwyer on Robert Moses's recommendation.

Writing in 1950, Farrell declared, "New York's public housing projects demonstrate that Negroes and whites can live together.

In 1950, during the Korean War, Farrell returned to active duty with the Army once more, and served with the Defense Production Administration.

In July 1951, he was transferred to the Atomic Energy Commission (AEC), the successor organization to the Manhattan Project, where he became the Assistant General Manager for Manufacturing.

The construction of new reactors at the Hanford and Savannah River Sites would eventually triple the production of nuclear weapons.

[50] Thomas graduated from West Point in the class of 1942, received the Silver Star Medal and the Distinguished Service Cross, and reached the rank of captain before being killed at Anzio on 25 February 1944.

[56] He served with the Army in the Vietnam War, where he commanded the 6th Battalion, 56th Air Defense Artillery during the Tet Offensive.

[57] Farrell's daughter, Barbara Vucanovich, was the first woman from Nevada to be elected to the United States House of Representatives, serving from 1983 to 1997.

[50][60] Ironically, the man who had spent a lifetime building things was principally remembered for the destruction of Hiroshima and Nagasaki.