(Due to arrive Canberra 13.4.29)
My dear P.M.,
An incident has occurred in connection with a paper on Coast Defence on which I have written a long letter to Henderson , the contents of which I have asked him to take an early opportunity of discussing with you. I have written it to him rather than direct to you for two reasons-firstly he can choose an appropriate moment to discuss it with you when you are rather less pressed than usual-and secondly it is well for him to be thoroughly impressed with the combination of tact and secrecy with which certain documents have to be treated. I will not burden you with the story here, other than to say that although the incident in itself is not of major importance, it is typical of possible incidents which might have much more serious consequences. I do not think it would be wise to make a real disturbance about this incident, but I think it would be appropriate to have a word with the Defence Minister  and General Chauvel  about it-although this of course is for you to decide.
It is unfortunate that this incident should have happened with regard to Belligerent Rights as this is a subject on which the War Office are particularly touchy. As it has been put to me by an officer in intimate touch with all the services, the War Office are fighting with their backs to the wall as they are very much afraid that they may lose a good deal of their grip on Coast Defence. Since the War, the War Office have lost the defence of Iraq and Aden, they are being pressed very hard by the Air Ministry with regard to the Defence of the Sudan and, to a lesser extent, with regard to the North-West Frontier of India. All this has meant an increase of prestige for the Air Ministry at the expense of the War Office-and the latter are naturally going to fight to the last ditch to maintain their previous position in respect of Coast Defence. This means, in short, that they will press their claim to defend ports by guns rather than let the Air have any finger in the pie by supplanting guns by torpedo carrying and other aircraft.
This is the reason why the War Office do not want any of their adumbrations to be known to anyone but themselves in case they should be used against them in the final struggle which will be staged at the C.I.D.
The danger of this incident is that it may make the War Office take the attitude in the C.I.D. that no Defence documents on War Office subjects (either C.I.D. or otherwise) should go to any Dominion other than through the Dominions Office or for the quarterly circular letter from the C.I.G.S. If they were to raise this matter officially at the C.I.D., it might result in a considerable curtailment of this valuable source of information on Defence matters.
However, my conscience is clear in the matter of this incident and Hankey  is quite satisfied and has appeased the War Office by a letter of explanation. But I think a word from you at the Australian end is rather called for.
I am, Yours sincerely, R.G. CASEY