In the spring of 1942, the US Navy could read a growing share of Japanese naval radio traffic. It knew a large operation was coming. It knew roughly how many ships. It did not know where.
The Japanese messages called the target “AF.”
Everything else depended on those two letters. The Pacific Fleet had three carriers it could put to sea. If they waited in the wrong place, Japan would take its objective unopposed. If the guess was right, the Americans could be waiting with the sun at their backs.
The basement and the argument
The guessing happened at Station HYPO, the Combat Intelligence Unit of the 14th Naval District, which worked in the basement of the Old Administration Building at Pearl Harbor. It was run by Commander Joseph J. Rochefort. His team had been attacking the Japanese fleet code, known to the Americans as JN-25, since before Pearl Harbor.
JN-25 was not a cipher you cracked once and read forever. It was built on a codebook of roughly 45,000 five-digit number groups. On a good day, HYPO recovered fragments of a message. Location names came through as short designators like AF.
Rochefort and the Pacific Fleet intelligence officer, Lieutenant Commander Edwin Layton, had seen “A” designators used for places around Hawaii. They believed AF was Midway, the atoll at the far western end of the Hawaiian chain.
Washington was not convinced. Analysts at OP-20-G, the Navy’s codebreaking headquarters, read the same traffic and leaned toward a strike in the North Pacific, possibly the Aleutians. Admiral Chester Nimitz could not move his carriers on a hunch from a basement while his own headquarters disagreed.
So HYPO decided to make the Japanese settle the argument themselves.
The broken water plant
The idea is credited to Lieutenant Commander Wilfred J. “Jasper” Holmes, a HYPO officer who before the war had been a submariner and a writer of magazine fiction. It was simple. Midway would report a problem in plain language, one that any Japanese listening post would want to pass along. Then HYPO would watch Japanese traffic to see which code name appeared next to it.
In mid-May 1942, with Nimitz’s approval, the commander at Midway was instructed to send an uncoded message saying the island’s water distillation plant had suffered serious damage and fresh water was needed.
On an atoll with no rivers, fresh water came out of that plant. A target that was going thirsty was exactly the kind of detail an invading force would care about.
The Japanese picked it up. Shortly afterward, HYPO intercepted a Japanese intelligence report stating that AF was short of water.
That was it. Midway’s plumbing problem now had a Japanese code name. The argument with Washington was over, and the Navy had told the enemy nothing it would miss: the distillation plant was working fine.
The same codebreaking work produced a date. HYPO concluded the attack would begin on June 3. Nimitz sent Enterprise, Hornet and the damaged Yorktown, which the Pearl Harbor yard had patched with around-the-clock repairs after the Battle of the Coral Sea, to a position northeast of Midway to wait.
Ten minutes on June 4
The Japanese did attack on June 3, as predicted, with bombers hitting the US forces there. The main event came the next morning.
On June 4, Vice Admiral Chuichi Nagumo’s four carriers launched against Midway’s airfields while American carrier aircraft hunted for them. The US attacks came in waves and mostly failed. Most of the torpedo planes were shot down before they could hurt anything. Then, at about 10:25, dive bombers from Enterprise and Yorktown arrived over the Japanese carriers at the same time.
Within minutes, Akagi, Kaga and Soryu were burning wrecks. The fourth carrier, Hiryu, escaped long enough to strike back at Yorktown, and was then found and bombed herself. She sank on June 5.
The fighting dragged on. Fleeing the area, the heavy cruisers Mogami and Mikuma collided while dodging an American submarine, and American aircraft caught the damaged pair on June 6. Mikuma sank. The photograph at the top of this page shows her that day.
The losses were lopsided. Japan lost four fleet carriers, a heavy cruiser and about 3,057 men, many of them experienced aviators. The United States lost Yorktown and about 307 men. Japan halted the carrier hull of the third Yamato-class battleship and turned it into Shinano to fill the gap.
Who got the credit
The question every account of Midway eventually gets to is who won it. The pilots, obviously. Nimitz, who took the risk. But none of them would have been in the right patch of ocean without the people in the basement.
What happened to Rochefort afterward is the part rarely mentioned in the movies.
Inside the Navy’s intelligence bureaucracy, HYPO’s success was an embarrassment to the people in Washington who had read the traffic differently. After the battle, complaints about the Hawaii station went to Admiral Ernest King. Rochefort was recalled. A recommendation that he receive the Distinguished Service Medal was turned down.
He finished the war commanding a floating dry dock in San Francisco. He never served at sea again.
It took decades, and pressure from retired intelligence officers, including CIA director William Casey, before the Navy reversed course. Rochefort was awarded the Distinguished Service Medal posthumously.
Holmes, whose water trick settled the question, went on to write a memoir of the codebreaking war, Double-Edged Secrets, published by the Naval Institute Press.
The water distillation plant that never broke is long gone. Midway Atoll is still there. It is now the Midway Atoll National Wildlife Refuge and Battle of Midway National Memorial, run by the US Fish and Wildlife Service, home to nesting albatross and currently closed to visitors. For the other side of the US fleet’s war that year, see the story of USS New Orleans at Pearl Harbor, which was at Midway too.
Sources
- The Battle of Midway, National Security Agency Center for Cryptologic History (via Wikisource)
- Midway, FDR Presidential Library and Museum blog, National Archives
- Station HYPO, Wikipedia
- Midway Atoll National Wildlife Refuge, US Fish and Wildlife Service
- W. J. Holmes, Double-Edged Secrets: U.S. Naval Intelligence Operations in the Pacific During World War II, Naval Institute Press, 1979
