During
World War II, he was public works officer at the Naval Operating Base in
Newport, Rhode Island., from 19 January 1942 to June 1943, where he supervised the construction of $100 million (equivalent to $ million in ) of naval facilities. After the war, he was awarded the
Navy Commendation Ribbon for this work. In 1944, he received the American Concrete Institute's Wason Medal for a paper on underwater concrete. From June 1943 to June 1944, he was responsible for closing out all the Navy's
cost plus fixed-fee contracts with the
Bureau of Yards and Docks. For his services, Halloran was awarded the
Legion of Merit. Halloran undertook the transformation of Tinian into an airbase for
Boeing B-29 Superfortress bombers. For this work his brigade had the 29th and 30th Naval Construction Regiments; a third regiment, the 49th Naval Construction Regiment, arrived in March 1945. The magnitude of the task was enormous. Two air bases were constructed:
North Field and
West Field. North Field alone had four runways, with of
taxiways, 265
hardstands, 173
Quonset huts and 92 other buildings. Its construction involved of excavations. In addition, the bases were provided with roads, fuel storage installations, hospitals and a deepwater harbor. For "outstanding service in charge of construction of an important base in the Pacific", Halloran was awarded an
oak leaf cluster to his Legion of Merit. He was promoted to the wartime rank of
commodore on 3 April 1945. ==Post-war==