Defense Files

Memorandum by the British Chiefs of Staff1

Probable Maximum Scale of Enemy Attack on West Coast of North America

Examination of Probable Form of Enemy Attach.

1. Enemy attack on the western seaboard of North America might be undertaken with the object of reducing United States offensive power in the Pacific through material destruction at shipyards and bases and also with the object of containing forces in America.

2. The following forms of attack have been considered:— [Page 266]

Seaborne expeditions;
Naval bombardment;
Carrier-borne air attacks;
Mine-laying in ports and their approaches and attacks by human torpedoes.

Seaborne Expeditions.

3. So long as a United States capital ship force is retained in the Pacific, it is extremely improbable that the Japanese would venture to launch a large scale expedition against a North American objective without battleship cover. It must also be assumed that apart from the destroyer escorts for the convoys, destroyers would be essential for A/A and A/S protection for the battleships.

4. The diagram attached to this Annex2 illustrates the large distances to be covered and shows that even if the enemy secured Dutch Harbour and Hawaii, it would be necessary to carry out the refueling of destroyers at sea.

While the refueling of an expedition at sea in hostile waters can not be ruled out positively, the complications and risks of such an operation on a large scale are sufficiently great to make it extremely unlikely. Added to this factor is the problem of maintaining a large scale expedition over 4,000 miles from its base.

5. These difficulties do not apply so strongly to a small scale raiding force escorted by cruisers and aircraft carriers which have much greater endurance. For this reason it is necessary to take into account the possibility that a force comprising 10–15 fast merchant ships carrying up to 2 brigades on a low scale of transport might undertake a destructive raid.

Naval Bombardment.

6. For the reasons given above, the employment of enemy battleships any great distance to the eastward of Hawaii is improbable. Naval bombardment of important objectives on the west coast of North America is therefore likely to be restricted to bombardment by armoured ships other than ships of the line and cruisers.

Carrier-Borne Air Attacks.

7. The employment of a carrier force escorted by cruisers for the air attack of important naval and industrial objectives is the most probable threat which has to be met. It is considered that the Japanese could make available a force of from 2–3 aircraft carriers escorted by 4 cruisers, without interfering with her dispositions for the other operations upon which she is now engaged. This would [Page 267] involve an attack by some 80–100 dive and/or torpedo bombers, with ship escort of approximately 24 fighters.

Mine-Laying in Ports and Their Approaches and Attacks by Human Torpedoes.

8. Mine-laying by raiders and submarines in coastal waters must be expected. The enemy’s ability to carry out attacks by human torpedoes has, it is understood, been demonstrated at Hawaii. It cannot therefore be ignored.


So long as the United States maintains a battle fleet in the Pacific, large scale seaborne expeditions against the western seaboard of North America and the employment of capital ship forces in this area are considered impracticable. The most probable enemy threat is carrier-borne air attacks and sporadic naval bombardment, but a small scale destructive raid cannot be ignored. In view of the great distances over which these operations would have to be undertaken, it is probably not necessary to provide a strong scale of defense except at selected points of great importance, which can be covered by the normal form of coast and air defense supplemented by mobile land and air striking forces.

  1. The memorandum was attached as Annex 2 to JOCSs–1; see ante, p. 90.
  2. The diagram attached to the source text as Appendix A is not printed. It shows mileages across the Pacific Ocean between such points as San Francisco, Hawaii, Tokyo, Jaluit, and Sydney.