Missing Pieces: - Royal Australian Navy

Facing The JaPanese OnslaughT 1941–42
reported by USN submarines, MacArthur’s air reconnaissance was his only real source
of information outside the coverage of his organic air. 219
The Japanese plan for Operation MO was multi-faceted, and only a broad description
will be necessary. While the Occupation Force sailed from Truk and rendezvoused
with the Attack Force from Rabaul on its way through the Jomard Passage in the
Louisiade Archipelago into the Coral Sea, the Striking Force would move to the east
of the Solomons with the idea of taking in the rear any Allied force attempting to
block the way to Port Moresby. Nimitz and his subordinate commanders hoped to
surprise the Japanese and eliminate the IJN carriers, using the combined air groups
of the two USN carriers in TF 17, and then to defeat their other forces by air and naval
bombardment and submarine attack. To do this required intelligence of a high order,
and, for the moment, this was available to the Allies through cryptanalysis and aerial
reconnaissance.
MacArthur, Nimitz, Fletcher, Leary, Prime Minister Curtin, CNS Royle and many other
senior figures had all or most of this intelligence. 220 The question becomes: what did
Rear Admiral Crace and his staff know? 221 It is not possible to say categorically that he
did not have the benefit of any Sigint on the Operation MO forces. He was not, at any
stage, invited onboard USS Yorktown for a conference with Fletcher, where some of the
latter’s Sigint information could have been shared verbally. He had been briefed by Rear
Admiral Brown onboard Lexington before an aborted attack on Rabaul scheduled for
3 March, but there is no mention of a similar courtesy extended by Fletcher prior to
the Battle of the Coral Sea. 222 However, Fletcher’s operation order, of which Crace,
Australia and Hobart were recipients, stated: ‘The Task Force is referred to daily radio
intelligence [this was the USN term for Sigint] promulgated by CSWPF’. 223 If Leary
was broadcasting Sigint it was in direct contravention of MacArthur’s instructions, but
this was a naval battle being waged by Nimitz. As well, the Australians would have
needed the appropriate cipher machine and keycards. There is no direct evidence
that they held these, although USN officers were attached to the Australian ships for
coding duties.
Crace had nothing equating to the radio intelligence teams reporting to Fletcher in
Yorktown and Lexington, and was thus unable to extract information from Japanese
transmissions. The RAN had taken the decision in 1938 to remove all its ‘WT Procedure
Y’ operators from ships and establish them in shore intercept stations. Crace was,
however, included on the collated all-source intelligence reports emanating from
CANF, mostly based on aircraft reconnaissance, from which he was able to make his
own assessment of the situation developing in the vicinity of the Louisiades. As the TF
17 air commander was to report: ‘Information furnished by shore and tender based air
was of considerable value strategically. Tactical information and attack support was
non-existent’. Crace and his staff would probably have agreed. 224
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