Let me now discuss the Japanese Navy’s important strategic code system, the JN-25.
It was 10 December, just three days after the attack on Pearl, that Washington gave a very dispirited Dungeon crew permission to join in on the attack on JN-25, a system that was a stranger to almost all of us. Unfortunately, a major change had been made in it on 4 December. The complicated, lengthy, and very difficult cipher applied to the code was changed. That was bad. But, fortunately, the code-book itself, which had been in use since 1 December 1940, was not replaced. That was good. In spite of the very limited success achieved with that system we did not want to have to start again completely from scratch. Washington and Corregidor had spent much time and effort getting that start on the codebook itself.

Corregidor was quick to make a break ‘ into the new cipher. They accomplished this in only four days — two days before we at Pearl received the go-ahead to start work. That fast break-in helped, soon the old-timers Rochefort, Finnegan, and Lasswell were making progress. We, the newcomers, were learning too. By the end of January things were moving right along. Progress was steady and we were obtaining significant bits of information in more and more messages. We could see we were becoming a team–a good team.

February brought more progress and more and larger fragments of information. We at Pearl were working our way up the line and we knew it. Wright, Dyer, and Holtwick were right in there, also, keeping a horrifying mass of code and cipher data as well as files of messages under usable control. They, too, were outstanding.

Admiral Ernest J. King

But not all was to remain sweetness and light. Before long discord came onto the scene. Washington and the Dungeon could not agree on some important items, including a difference of opinion regarding predicted Japanese operations being discussed in JN-25. In a disagreement of this type Washington had one very important advantage. They had personal access to the ear of the all-powerful, very determined, and extremely unforgiving super boss, Admiral Ernest J. King. Most happily the Rochefort – Lasswell-Layton, etc., team was seldom caught off base — in fact, they almost always were right on the proper wave length, a fact that was not lost on Admiral Nimitz. But the problem did not go away.

The bits of operational intelligence we did, even at that time, succeed in pulling out of JN-25 sometimes were verifiable, as in the case when we could put a submarine on the track of the carrier KAGA. KAGA was returning to Japan for repairs after hitting a shoal off Malay. Unfortunately the submarine skipper could not approach close enough to fire torpedoes, but this and other analogous cases soon proved that we were not just putting out dangerous guesswork — nor even guesswork.

Operational bits continued to improve — they became more frequent and the readable text was usually longer.

Then came April and the Dungeon gang was hotter yet. It was in the middle of April that Admiral King — very unusually – requested Rochefort to send him an estimate of the enemy’s probable intentions over the next two months or so. There isn’t time to go into that here other than to say that some Japanese language war history books I have read use Rochefort’s reply as valid reason to rank him among the great prophets of all time.

07But along with April also came something that just never should have been — the 18 April Doolittle raid on Japan – usually called a raid on Tokyo. Carriers HORNET and ENTERPRISE had been sent on a foolish grandstand mission when they should have been heading for the Coral Sea.

But this patently foolish mission immediately became a blessing. The Japanese high command vastly-over-reacted to this loss of face in several important ways that furthered the Allied cause tremendously. It even furthered the cause in the Dungeon, where greatly increased volume of traffic, much of it from units normally sending few dispatches, gave us a line-up on Japanese fleet organization far superior to anything we had worked out previously. This would help tremendously with our intelligence for Midway.

It was now obvious to Rochefort and to Layton, and hence to Nimitz, that an important move southward from Rabaul, the major Japanese base northeast of Australia and northwest of the Solomons Islands probably to the important Australian port and army and air base at Port Moresby in Southeastern New Guinea, was in the offing. An accompanying move to occupy a potential seaplane base at the island of Tulagi in the Solomons also appeared to be scheduled in the same operation. The information gleaned from JN- 25 as to the forces and the time schedule for this move paid off handsomely. Nimitz was able to get Carrier Task Forces 16 and 17 with carriers LEXINGTON and YORKTOWN on the scene in time for what is now known as the Battle of the Coral Sea — the first carrier versus carrier battle in history.

The results were not all we might have wished. But in the engagements on 7 and 8 May the Japanese threat to Port Moresby was stopped; however we lost LEXINGTON, and the carrier YORKTOWN suffered uncomfortable damage.

A really valid story of the Battle of the Coral Sea has yet to be told. I wish I might say more on that —–

Admiral Halsey

Before the end of April the Japanese navy gave strong indications that it was interested in the Aleutian Islands, and Joe Rochefort was seeing additional evidence that another move might be made in the central Pacific. Washington now was very insistent upon protecting the important points of Samoa, New Caledonia, and Fiji on the vital lifeline to Australia. Admiral Halsey with carriers HORNET and ENTERPRISE had failed to get to the Coral Sea on time for that battle but now were to be left in the Southwest Pacific area. These were Admiral King’s wishes-and wishes of Admiral King were akin to the Ten Commandments chiseled in stone. Could Admiral Nimitz dare call them back to Hawaii?

During all these days of late April and early May both Finnegan and Lasswell were magnificent and Rochefort was superb. Rochefort knew his team. And they knew him. And they understood each other. Finnegan would make fearful quantum leaps. Fortunately, he was almost always right. When he wasn’t right the steadier, smoother moving Lasswell usually would be the one to pull him and his thoughts, not too abruptly, back to the realm of reality. Our work on JN-25 was hot.

Source: http://www.usspennsylvania.com/TheDungeon.htm