Despite the Americans’ frustrations in 1942, the Japanese were more vulnerable than it might have seemed. It was one thing to capture so many islands and bases, and another thing to supply and defend them. The U.S. Congress had passed the Two-Ocean Navy Act, funding a massive fleet-construction program. And it was important that America’s aircraft carriers had been safely out of harm’s way during the attack on Pearl Harbor. World War II was the first war whose naval outcome would turn on aircraft carriers and the planes flying off their decks.
Things had begun looking up for the code breakers just before the women’s arrival in June. The first indication of the vital role code breaking would play in the outcome of many epic Pacific engagements had begun to emerge in early May, when decoded JN-25 messages tipped off Admiral Nimitz that a Japanese fleet was steaming back from an assault on the British in Ceylon and now aimed to capture Port Moresby in New Guinea. The Japanese were astonished when two carrier task forces of the American Navy materialized to meet them. The Battle of the Coral Sea, from May 4 to May 8, 1942, was the first naval battle in which the opposing ships never saw each other—the fighting was all done by aircraft—and it was the first Pacific contest where code breaking played a key role in the outcome. The result was a tactical draw—the Americans lost the Lexington, and the Yorktown was badly damaged, but the Japanese losses, including many of its best-trained pilots, were bad. It also checked Japanese expansion toward Australia.
By mid-May, the U.S. Navy got wind of an even bigger Japanese operation. Thousands of messages began flashing back and forth in JN-25, suggesting Japan was sending a massive flotilla somewhere. “Vague indications of an operation to be launched,” noted one early message. Joe Rochefort, one of many officers working in the Pacific who had been trained by Agnes Driscoll, supervised the Pearl Harbor code-breaking team. A lot of information was obtained about the planned Japanese operation, but there was one key puzzle piece that stumped everybody: In mid-May the Americans intercepted a message saying that the Japanese, who often used two-letter geographic designators, were headed to “AF.” The code breakers had recovered some other designators but could not be certain where AF was. Rochefort and his team felt sure AF stood for Midway, a tiny atoll where the United States still maintained a base, crucial for defending Hawaii and the West Coast and indeed for maintaining any U.S. presence in the Pacific. Others thought the target might be Hawaii or the Aleutians.
So Rochefort and Edwin Layton, Nimitz’s chief intelligence officer—also trained by Agnes Driscoll—hatched a plan. They instructed the men at the Midway base to radio a message—not coded, just plain English—saying their distillation plant had broken down and that Midway was short on water. The idea was that the Japanese would intercept the bulletin and pass it on. Just as they hoped, a local Japanese unit picked it up and sent its own message saying AF was short of water, and the message got passed along to the fleet. The Americans intercepted it. The trick succeeded. The Americans had confirmed that AF stood for Midway.
Admiral Yamamoto’s aim was to achieve the knockout blow that had eluded him at Pearl Harbor. Mustering a mighty flotilla of more than two hundred warships, transports, and auxiliaries, Yamamoto intended to split his forces and send one contingent—a smaller one—to the Aleutian Islands, off the mainland of Alaska, to mount a subsidiary attack that would serve as a decoy. Nimitz, he reckoned, would hasten to counter that attack. By the time Nimitz got back, the Japanese would be at Midway in great numbers, prepared to ambush him and finish him off.
But Nimitz didn’t take the bait. He let the Japanese head to the Aleutians and set about reinforcing Midway, in order to ambush the ambushers. Thanks to cryptanalysts reading JN-25, Nimitz knew more about the planned attack than most Japanese officers did. “He knew the targets; the dates; the debarkation points of the Japanese forces and their rendezvous points at sea; he had a good idea of the composition of the Japanese forces; he knew of the plan to station a submarine cordon between Hawaii and Midway,” noted an internal history.
The Japanese duly showed up on June 4. Their carrier task force launched an air strike on the island, but this was no Pearl Harbor: American fighters scrambled to meet the incoming planes in midair, taking heavy fire but pushing the enemy back while four waves of U.S. bombers took off toward the Japanese carriers. The Japanese had expected to be attacked by planes from Midway, but didn’t realize the Americans also had aircraft carriers nearby. They soon would: From the decks of the Hornet, the Yorktown, and the Enterprise, torpedo and dive bombers took off. The American planes caught the Japanese carriers as they were preparing for another attack on Midway; the Japanese decks were crowded with bombs and fuel hoses, setting off massive fires. The Japanese had expected a quick victory; by the end of the first day, with its attack force stunningly crippled, it was clear this would be a different story. By the time the Japanese withdrew, calling off the operation, the U.S. Pacific Fleet had lost 2 ships, 145 aircraft, and 307 men; the Japanese endured devastating losses including 4 carriers, almost 300 aircraft, and more than 2,500 men.
The four-day Battle of Midway was an unparalleled American victory. One of the most storied naval battles in world history, it marked the end of Japan’s expansion in the Pacific and a major turning point in the war. Slowly but surely, for the United States, the Pacific War would turn from a defensive to an offensive one. The fact that an outnumbered American fleet had scored a resounding win over an armada of enemy attackers did much to lift naval morale—not to mention the spirits of the country. Commanders are often loath to share credit, but Nimitz allowed that code breaking had provided a “priceless advantage” at Midway. To be sure, it was mostly a victory for the code breakers in the Pacific unit; Washington was still bogged down in acrimony between code breakers and top brass. Even so, “the Battle of Midway gave the Navy confidence in its cryptanalytic units,” an internal history noted, and it gave the code breakers confidence in themselves. More staff would be funneled into Washington, more funds freed up, more teletype lines established. The Americans had delivered payback for Pearl Harbor.
“Midway,” the history noted, “was a vindication and an incentive.”
The Midway victory also set in motion one of history’s great bureaucratic backstabbings. Joseph Wenger and John Redman, two of the top intelligence officers in Washington, had believed the attack would happen a week later than it did. Joe Rochefort and the team in Pearl Harbor had gotten the date right. To cover their mistake, the Washington bigwigs (who feared Rochefort was building a unit to compete with theirs) let it be known that they were the ones who had pinpointed the correct date, and Pearl Harbor had gotten it wrong. This shocking lie found its way all the way up to Ernest King, commander in chief of the U.S. Fleet. Redman and Wenger continued to conspire against Rochefort, who eventually was relieved of his command and put in charge of a dry dock.