Midway had been occupied by Americans since the early twentieth century, and in the build-up to World War II in the Pacific, the U.S. military had established a significant presence on the island. It had air strips, gun emplacements and a sea plane base; the channel had been widened, and it was a vital resupply point for naval vessels and submarines. Midway was second only to Pearl Harbor in its importance to the U.S. Navy.
After the Doolittle Raid on the Japanese home islands on Apr. 18, 1942, the Japanese drastically altered their naval campaign in the Pacific. Both Japanese air and naval assets were recalled closer to home in order to deter further attacks. The Japanese military had promised that the home islands would never come under attack. America had proven it wrong and therefore had to be dealt with decisively. Admiral Isoroku Yamamoto decided that he would surprise the Pacific Fleet with an attack on Midway, and then crush it while it attempted to defend the island.
The Japanese plan utilized a three-pronged approach. Admiral Chuichi Nagumo’s carrier force would approach Midway from the northwest and suppress the island’s defenses. Behind Nagumo’s force, Admiral Yamamoto would bring a battleship force. Finally, Admiral Nobutake Kondo would bring the actual invasion and occupation force to the island from the west and southwest. In addition, the Japanese would first feint an invasion 3,000 miles away in the Aleutian Islands.
The Pacific Fleet knew of Japan’s plan thanks to the Pacific Fleet Combat Intelligence Officer, Captain Edwin T. Layton, and the Navy’s cryptologic capability. Layton was a huge proponent of cryptology, although it still was looked upon with suspicion by many operational commanders. In 1942, the Navy had three cryptologic stations decoding Japanese messages: one each in Corregidor, Washington DC, and Hawaii. The Hawaii station, code-named “HYPO,” had been working on deciphering a little-used, administrative Japanese Flag cipher. On Dec. 8, 1941, it was authorized to assist the other two stations in decrypting the Japanese fleet code, JN-25B.
Station HYPO, along with the Station CAST on Corregidor, began successfully deciphering Admiral Yamamoto’s plan in early March 1942. CAST had decoded Japanese military messages about Midway and the Aleutian Islands. On May 1, military activity in Japan verified that forces were being assigned to these objectives. By this point, Navy cryptologists had accurately deciphered the who, what, and where, but they still did not have the when. The real breakthrough came on May 25, when Station HYPO successfully discovered the Japanese date cipher. The Japanese would proceed with its feint in the Aleutians on June 3, and the attack on Midway would begin on June 4.
Despite objections from several others on his staff, Admiral Chester Nimitz decided to base his operational timetable on these dates. As predicted, on Jun. 3 the Aleutian Islands were attacked. Shortly afterwards, a U.S. patrol plane sighted the landing and occupation forces of Admiral Kondo 700 miles away from Midway. In the early morning hours of Jun. 4, Admiral Nimitz asked Captain Layton to tell him when and where he predicted the Fleet would first spot the Japanese carrier force. Layton replied, “6 a.m., at a bearing of 325 degrees and 175 miles from Midway.”
Shortly before 6 a.m. on Jun. 4, a patrol plane spotted the Japanese carrier force 180 miles from Midway, at a bearing of 320 degrees. Upon hearing this report, Admiral Nimitz turned to Captain Layton and said, “Well, you were only five minutes, five degrees, and five miles off.”
The Japanese suffered the loss of four carriers, one heavy cruiser, and all of their aircraft in the Battle of Midway. They had lost naval superiority in the Pacific, and their larger strategic goals of expansion in the region were forestalled. After the Battle of Midway, the Japanese never regained momentum and remained on the defensive through three more years of fighting in the Pacific.