Dating back to the beginning of the 20th century, Oʻahu held a position of primary importance in the military structure of the US before and during WWII. During the prewar years Oʻahu and the Panama Canal Zone were the two great outposts of continental defense. (army-mil)
A key goal in the Pacific was to hold Oʻahu Island as a main outlying naval base and to protect shipping in the waters around the Hawaiian Islands.
In January 1905, President Teddy Roosevelt instructed Secretary of War William H Taft to convene the National Coast Defense Board (Taft Board) “to consider and report upon the coast defenses of the United States and the insular possessions (including Hawai‘i.)”
In 1906 the Taft Board recommended a system of Coast Artillery batteries to protect Pearl Harbor and Honolulu. Between 1909-1921, the Hawaiian Coast Artillery Command had its headquarters at Fort Ruger and defenses included artillery regiments stationed at Fort Armstrong, Fort Barrette, Fort DeRussy, Diamond Head, Fort Kamehameha, Kuwa‘aohe Military Reservation (Fort Hase – later known as Marine Corps Base Hawaiʻi) and Fort Weaver.
The Army mission in Hawai‘i was defined as “the defense of Pearl Harbor Naval Base against damage from naval or aerial bombardment or by enemy sympathizers and attack by enemy expeditionary force or forces, supported or unsupported by an enemy fleet or fleets.”
The District was renamed Headquarters Coast Defenses of Oʻahu sometime between 1911 and 1913. Following World War I and until the end of World War II, additional coastal batteries were constructed throughout the Island.
Then, on December 7, 1941, the Japanese attacked Pearl Harbor.
As soon as the air attack was over, the Hawaiian Department plunged into a reconstruction and new construction effort of unprecedented scale and pace.
By December 12, the Army position was “to take all possible steps short of jeopardizing the security of the Continental United States and the Panama Canal to reinforce the defenses of Oʻahu.”
Wartime reality hit the neighbor islands a few days later. A group of about nine Japanese submarines were kept in the vicinity of Hawaiʻi until mid-January – they were stationed there to find out just how much damage had been done to the American military.
Just before dusk on December 15, a submarine lobbed about ten shells into the harbor area of Kahului on Maui, and three that hit a pineapple cannery caused limited damage.
Over a 2½-hour period during the night of December 30 – 31, submarines engaged in similar and nearly simultaneous shellings of Nawiliwili on Kauaʻi, again on Kahului, Maui and Hilo on the Big Island.
The principal immediate change in Hawaiʻi’s defense structure came about on December 17, 1941, when the top Army and Navy commanders were replaced and all Army forces in the Hawaiian area were put under command of the Commander in Chief, Pacific Fleet.
Under a cooperative agreement, the army operated coast defense guns, all anti-aircraft batteries except those on naval ships, most of the pursuit aircraft on the Island, an inshore patrol which extended 20-miles out to sea and aircraft warning service. The navy operated the fleet and distance reconnaissance extending to 600-miles out to sea.
Two arguments won the approval of the War Department during December for a much larger reinforcement of Hawaiʻi. The Navy contended that the sure defense of the Hawaiian area depended primarily on Army air power and that the security and effectiveness of that air power required its dispersion among the major islands of the Hawaiian group.
Secondly, while the immediate reinforcement of December 1941 might ensure against a direct attempt by the enemy to invade Oʻahu, the Japanese had the naval strength to cover an invasion of one or more of the almost undefended neighbor islands. From bases on these islands the enemy could attack and possibly starve out Oahu.
These arguments led to plans for garrisoning the other islands of the Hawaiian group. And, Hilo was a natural choice.
After the sugar industry developed across the Islands, Hilo grew to be the second largest town in the islands, acting as a business hub for the numerous plantations along the Hilo-Hamakua coast, as well as a transport center for incoming supplies and equipment and outgoing crops.
In 1908, construction began on the Hilo Bay breakwater along the shallow reef, beginning at the shoreline east of Kūhiō Bay; by 1929 the breakwater was completed and extended roughly halfway across the bay. Piers were built and extended by 1927.
(Contrary to urban legend, the Hilo breakwater was built to dissipate general wave energy and reduce wave action in the protected bay, providing calm water within the bay and protection for mooring and operating in the bay; it was not built as a tsunami protection barrier for Hilo.)
In 1926, a 400 by 2,000-foot field had been cleared for Hilo Airport and on February 11, 1928, the new airport was dedicated. A second and third runways were added and the airport was renovated (the renovation dedication ceremony was held May 2, 1941.)
At the outbreak of World War II, Hilo Airport was taken over by the Army Engineers, and an Air Corps fighter squadron was stationed there. US Army Engineers constructed military installations and continued the expansion of runways, taxiways and parking aprons. The name of Hilo Airport was changed to General Lyman Field on April 19, 1943.
At Hilo, a mobile field battery of 155-mm guns was set up in December 1941. Four 4-inch naval guns were later emplaced in 1942.
To help man them, the 96th Coast Artillery Regiment (AA) (Semi-mobile – activated April 15, 1941 at Camp Davis, North Carolina, and trained there until December 27) arrived in Hilo on March 10, 1942. (They stayed at Hilo until December 1943; then they transferred to Oʻahu.)
The Hilo battery was abandoned in 1945. (Lots of information here is from army-mil.)