JAN. 24, 1941.
Serial 09112


MY DEAR MR. SECRETARY: The security of the U. S. Pacific Fleet while in Pearl Harbor, and of the Pearl Harbor Naval Base itself, has been under renewed study by the Navy Department and forces afloat for the past several weeks. This reexamination has been, in part, prompted by the increased gravity of the situation with respect to Japan, and by reports from abroad of successful bombing and torpedo plane attacks on ships while in bases. If war eventuates with Japan, it is believed easily possible that hostilities would be initiated by a surprise attack upon the Fleet or the Naval Base at Pearl Harbor.

In my opinion, the inherent possibilities of a major disaster to the fleet or naval base warrant taking every step, as rapidly as can be done, that will increase the joint readiness of the Army and Navy to withstand a raid of the character mentioned above.

The dangers envisaged in their order of importance and probability are considered to be:

(1) Air bombing attack.
(2) Air torpedo plane attack.
(3) Sabotage.
(4) Submarine attack.
(5) Mining.
(6) Bombardment by gun fire.

Defense against all but the first two of these dangers appears to have been provided for satisfactory. The following paragraphs are devoted principally to a discussion of the problems encompassed in (1) and (2) above, the solution of which I consider to be a primary importance.

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Both types of air attack are possible. They may be carried out successively simultaneously, or in combination with any of the other operations enumerated The maximum probable enemy effort may be put at twelve aircraft squadrons and the minimum at two. Attacks would be launched from a striking force of carriers and their supporting vessels.

The counter measures to be considered are:

(a) Location and engagement of enemy carriers and supporting
vessels before air attack can be launched.
(b) Location and engagement of enemy aircraft before they reach their
(c) Repulse of enemy aircraft by anti-aircraft fire;
(d) Concealment of vital installations by artificial smoke;
(e) Protection of vital installations by balloon barrages.

The operations set forth in (a) are largely functions of the Fleet but, quite possibly, might not be carried out in case of an air attack initiated without warning prior to a declaration of war.

Pursuit aircraft in large numbers and an effective warning net are required for the operations in (b). It is understood that only thirty-six Army pursuit aircraft are at present in Oahu, and that, while the organization and equipping of an Anti-Air Information Service supported by modern fire control equipment is in progress, the present system relies wholly on visual observation and sound locators which are only effective up to four miles.

Available Army anti-aircraft batteries appear inadequate if judged by the standards of the war in Europe. There are now in Oahu 26 3" fixed anti-air craft guns (of which something over half are grouped about Pearl Harbor), 56 mobile 3" guns, and 109 .50 caliber machine guns. The anti-aircraft batteries are manned in part by personnel which is also required to man parts of the sea coast artillery. Should an attack on Oahu combine air attack with a gun bombardment, one or the other countering fires would suffer from lack of men. If the prevailing high ceiling is taken into account the caliber of the anti-aircraft guns might be inadequate against high altitude bombing attack.

By late summer the defenses will be considerably strengthened by additions in guns, planes, and radio locators. It is understood, sixteen additional 3" Mobile twenty-four 90 mm., and one hundred twenty 37 mm. guns will be on hand, the pursuit aircraft strength is to be expanded to a total of 149; the new radio locators will have an effective range of 100 miles. Although the caliber of the guns will still be small for effective action against high altitude bombers, this augmentation will markedly improve the security of the Fleet. It does not, of course, affect the critical period immediately before us.

The supplementary measures noted in (d) and (e) might be of the greatest value in the defense of Pearl Harbor. Balloon barrages have demonstrated some usefulness in Europe. Smoke from fixed installations on the ground might prove most advantageous.

To meet the needs of the situation, I offer the following proposals:

(1) That the Army assign the highest priority to the increase of pursuit aircraft and anti-aircraft artillery, and the establishment of an air warning net in Hawaii.

(2) That the Army give consideration to the questions of balloon barrages, the employment of smoke, and other special devices for improving the defenses of Pearl Harbor.

(3) That local joint plans be drawn for the effective coordination of naval and military aircraft operations, and ship and shore anti-aircraft gun fire, against surprise aircraft raids.

(4) That the Army and Navy forces in Oahu agree on appropriate degrees of joint readiness for immediate action in defense against surprise aircraft raids against Pearl Harbor.

(6) That joint exercises, designed to prepare Army and Navy forces in Oahu for defense against surprise aircraft raids, be held at least once weekly so long as the present uncertainty continues to exist.

You concurrence in these proposals and the rapid implementing of the measures to be taken by the Army, which are of the highest importance to the

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security of the Fleet, will be met with the closest cooperation on the part of the Navy Department.

Sincerely yours,
                                                         /s/ Frank Knox
The Honorable The SECRETARY OF WAR.