In the spring of 1942, Admiral Ernest J. King, Commander in Chief (COMINCH), U.S. Fleet was waging a war on two fronts. The U.S. and Allies had adopted a strategy of “Europe first.” Though King had many concerns about Japan’s exploits in the Pacific, all of which he shared regularly with Nimitz, he was somewhat preoccupied with the German submarine threat in the Atlantic. German U-boats were wreaking havoc on U.S. and British merchant vessels carrying crucial war supplies to Allied forces in Europe. Nevertheless, he could not allow Japanese advances in the southwest Pacific to go un-checked. He was particularly concerned with the defense of Australia. The IJN’s recent attacks and occupation of Rabaul, were followed by an air campaign focused on softening the defenses at Port Moresby on the southwest coast of New Guinea — the subsequent occupation of which would be certain to threaten the security of Australia.
Piecing Together the Puzzle
Though Japan’s pounding of Port Moresby had certainly raised concerns, there were no definitive indications that it would be Japan’s next target. Elliot Carlson writes: “By the end of March, aside from bombing Pearl Harbor, Japanese forces had occupied British Borneo and all of the Dutch East Indies. Their army was advancing in Burma and was about to complete the conquest of the Philippines. Japanese warplanes continued to strike Moresby, while farther west a large IJN armada was believed to be heading toward the Indian Ocean. The U.S. Navy had no very good idea where the Japanese were going to strike next.” This bafflement was not limited to war planners alone. The picture in the dungeon was similarly obscured — though only for a time.
Beginning in late March and early April, intercepts collected by Oahu and Belconnen listening posts began to indicate an impending operation far from the Indian Ocean where Nagumo’s kido butai was currently operating. Two intercepts on 25 March and 8 April both specifically referenced an “RZP campaign.” The latter of the two messages further indicated the participation of one of Nagumo’s six carriers, the Kaga. A key piece of the puzzle that was missing was the specific identity, or location, of “RZP.”
RZP was one of many digraphs/trigraphs that the IJN had been using to designate specific geographic locations throughout the Pacific. Cryptanalysts at Stations CAST, HYPO and Belconnen had long been working to equate each intercepted designator with a corresponding geographic location. By early April, cryptanalysts had not only uncovered the identity of many individual designators, but they had also begun to recognize patterns. Digraphs beginning with the letter “A” were determined to be associated with U.S. controlled areas in the central and northern Pacific. Designators beginning with the letter “R” had proved to indicate Australian locations of interest. Though many “R” designators had been correlated, “RZP” was listed as unknown. Rochefort believed it to be Moresby.
On 8 April, CDR Rochefort sent a message to COMINCH, CINCPAC, and OP-20-G telling them just that: “RZP is Moresby.” Carlson writes: “To arrive at that conclusion, Rochefort brought to bear all his analytic skills. As was his practice by now, he reached his verdicts by a combination of deduction and guesswork and an uncommon ability to fill in blanks.” Though similarly skilled cryptolinguists could arrive at completely different conclusions analyzing the exact same messages, future intercepts would confirm Rochefort’s assertion.
Operation Order No. 1
One week after determining that the Kaga would be participating in the RZP campaign, HYPO’s analysts intercepted messages indicating possible participation by three additional carriers: the light carrier Shoho and the two huge fleet carriers of Carrier Division 5, Shokaku and Zuikaku. This revelation was the result of traffic analysis and still involved some guesswork. Intercepts had indicated that each of the three carriers, currently operating in Empire waters off Yokosuka, had significantly increased communications with Japanese positions in Truk and Rabaul. Though message content never revealed such, HYPO’s traffic analysts concluded that the only possible reason for such communication was an impending offensive in the southwest Pacific.
Further evidence — and confusion — came on 24 April in a message that referenced a previously unheard codeword “MO.” The dungeon had intercepted a message from Vice Admiral Inoue assigning special callsigns to his invasion force. These callsigns revealed the structure of Inoue’s fleet which would consist of multiple attack and occupation forces for MO, RZP, RXB, and RY. Two of the four designators were known. RZP was Moresby and RXB was Tulagi. RY was suspected to be in the Gilberts, but it was the MO designator that caused the greatest concern. It had never been intercepted and it did not follow the same pattern as the others. Nevertheless, Rochefort believed it to be yet another code for Moresby.
Less than a week later, confirmation came in the form of Operation Order No.1. Up to this point, Rochefort had arrived at all of his conclusions based on indications provided primarily by traffic analysis. The intercept and translation of Yamamoto’s Operation Order No. 1 by HYPO’s cryptanalysts validated all previous estimates. The order stated: “The objective of the MO will be first, to restrict the enemy fleet’s movements and will be accomplished by means of attacks on outlying units and various areas along the north coast of Australia. The Imperial Navy will operate to its utmost until this is accomplished.” Armed with this information, Admiral Nimitz made preparations to meet the Japanese fleet in the Coral Sea.
Japan’s attack forces were expected to comprise four carriers — Kaga, Shokaku, Zuikaku, and Shoho. However, this estimate was partially flawed in that HYPO’s analysts had incorrectly assessed that the CarDiv 5 carriers would be in addition to the Kaga and Shoho, when in fact, they were directed to relieve the Kaga. At the time of the battle, the Japanese force would consist of two fleet carriers and one light carrier along with one cruiser division and the warplanes of the Eleventh Air Force.
Nimitz had two carrier task forces at his disposal to counter the Japanese offensive — Task Force 11, commanded by Rear Admiral Fitch aboard USS Lexington, and Task Force 17, commanded by Rear Admiral Fletcher aboard USS Yorktown. Fletcher was placed in command of the overall operation. In addition to the two carriers, the combined task force comprised six cruisers and 14 destroyers as well as three tankers. Fletcher would be outnumbered.
As forecasted by Rochefort and his team, Admiral Inoue’s forces attacked and invaded Tulagi on the 3rd and 4th of May 1942. During the attack, aircraft from USS Yorktown sunk or damaged several Japanese warships — alerting Inoue to the presence of U.S. forces in the area. In the days that followed, carriers from both sides would engage for the first time since Japan’s attack on Pearl Harbor six months earlier. On 7 and 8 May, Fletcher’s and Inoue’s forces met in the Coral Sea in what would become a historic battle. The U.S. sank the Japanese light carrier, Shoho, and severely damaged the huge fleet carrier, Zuikaku. The Japanese damaged Yorktown and destroyed Lexington. In addition to ships sunk or damaged, both sides incurred heavy aircraft losses as well. On the 8th of May, having failed to invade Port Moresby, Admiral Inoue recalled his forces. What would be recorded in the history books as the Battle of the Coral Sea was the first engagement of two opposing aircraft carriers as well as the first in which neither ships sighted nor fired directly upon the other.
Though a tactical loss for Fletcher’s force, the Battle of the Coral Sea was a strategic victory for the U.S. Pacific Fleet. Nimitz had stopped Yamamoto’s attempt to further strengthen the Empire’s position in the South Pacific. The battle was also significant in that it not only destroyed one of Yamamoto’s carriers, but it also depleted the aircraft from the fleet carrier Zuikaku which would have significant implications for future battles.
As the War in the Pacific waged on, Rochefort and his team in the dungeon were proving themselves as key enablers for success against the IJN. Rochefort’s keen analysis of Japanese movements and intentions enabled Admiral Nimitz to put the right forces in the right place at the right time. His ability to do so again would be tested in just a few short weeks.
*Stay tuned next week for the fifth and final installment of the “Path to Midway” series: “A Glorious Page in Our History.”
 Carlson, Elliot. Joe Rochefort’s War: The Odyssey of the Codebreaker Who Outwitted Yamamoto at Midway. Annapolis, MD: Naval Institute Press, 2011. Print. [pp. 91-113]
 Symonds, Craig. The Battle of Midway. New York, NY: Oxford University Press, 2011. Print.
 Toll, Ian. Pacific Crucible: War at Sea in the Pacific, 1941-1942. New York, NY: W. W. Norton & Company, 2012. Print.