In light of some information, I suppose it can be argued that UK might not declare a general war with Japan, should the latter attack DEI.
On receipt of the Chiefs of Staff Appraisal, Admiral Wenneker, Germany’s Naval Attaché to Japan sent the following summary to Berlin.
Despatch of cipher Tel, Nr 209/40 gKdos to 212/40 gKdos to Navy High Command
For Naval War Staff:
Of note amongst materials reaching here on 6 December from Ship ‘16’ is a captured report of the War Cabinet about the situation in the Far East in the event of Japanese intervention against Britain, dated 15 August:
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In present situation we (i.e. Britain) are unable to send fleet to Far East.
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Japan needs Singapore in order to meet its ambitions.
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Until the situation in Europe clarified itself, open breach between us and America and Japan improbable.
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We must avoid ‘open clash’ with Japan in order to gain time and promote military co-operation.
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In the absence of Fleet, we are unable to prevent damage to our own interests. We should retire to a base from which it will be possible to restore our position later on.
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Four possible Japanese attacks:
(a) Direct attack
(b) Advance into Indo-China or Siam.
© Attack on Dutch East Indies.
(d) Attack on Philippines.
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Japanese first step either Indonesia or Siam; then Dutch East Indies before Singapore.
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In current situation we would put up with Japanese attack on Siam or Indo-China without going to war.
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In event of Japanese attack on Dutch, and they offered no resistance, no war between us and Japan. But if Dutch resist, then they would have our full military support.
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Hong Kong without any significance and cannot hold out for long without presence of a substantial fleet. However, will be held as long as possible.
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Strategy in Event of War:
a) Impossible to prevent Japanese gaining access to Indian Ocean.
b) We cannot maintain naval lines of communication with Northern Malaya.
c) Hope to maintain bulk of commerce by sea with Suez and eastern Australia.
d) Apart from cruiser raids, Japanese attack on Australia improbable without first taking Singapore.
e) Japanese occupation of Suva and Fiji likely in order to use as a base.
f) Need to defend all of Malaya and not just Singapore Island.
g) Holland probably willing to prepare joint plan for defence of Dutch Indies. In view of limited assistance we can give, their help unlikely if British territory attacked. Consequently, not desirable to begin Staff Talks at this time.
h) As long as no fleet, forced to turn to air force. But this can only be provided in a limited way. Therefore,
strong land forces necessary in Malaya. Concentration there a top priority.
i) Borneo indefensible. Very little air power available for protection of shipping in Indian Ocean.
j) Until situation in Europe improves, Far East gravely threatened, especially if subjected to determined
Japanese sorties, with heavy naval units.
- Our construction programme never intended to cover war with Germany, Italy and Japan, simultaneously.
The only hope for providing a fleet for the Far East based on early, successful attack on Italy and the
Mediterranean
- Objectives we must seek to achieve.
a) Commonwealth must send one Division to Malaya.
b) By end of 1940, two fighter squadrons and two squadrons of long-range (?) land aircraft to be sent to Far East.
c) Naval construction programme to be accelerated.
d) Withdrawal of Garrison from North China and Hong Kong.
e) New Zealand must send one brigade to Fiji.
f) General Staff consultations with Holland once situation in Malaya improves.
- Detailed information on the strengths of land, sea and airforces in the Far East.