PEARL HARBOR, Hawaii — Six months after the devastating attack on Pearl Harbor, Japan sent four aircraft carriers to the tiny Pacific atoll of Midway to draw out and destroy what remained of the U.S. Pacific Fleet.

But this time the U.S. knew about Japan’s plans. U.S. cryptologists had cracked Japanese communications codes, giving Fleet Commander Adm. Chester Nimitz notice of where Japan would strike, the day and time of the attack, and what ships the enemy would bring to the fight.

The U.S. was badly outnumbered and its pilots less experienced than Japan’s. Even so, it sank four Japanese aircraft carriers the first day of the three-day battle and put Japan on the defensive, greatly diminishing its ability to project air power as it had in the attack on Hawaii.

On Monday, the current Pacific Fleet commander, Adm. Cecil Haney and other officials flew 1,300 miles northwest from Oahu to Midway to mark the 70th anniversary of the pivotal battle that changed the course of the Pacific war.

“After the battle of Midway, we always maintained the initiative, and for the remaining three years of the war, the Japanese reacted to us,” Vice Adm. Michael Rogers, commander of the U.S. Fleet Cyber Command, told a crowd gathered outside Nimitz’s old office at Pearl Harbor on Friday to commemorate the role naval intelligence played in the events of June 4-7, 1942.

“It all started, really, in May of 1942 with station Hypo (the Combat Intelligence Unit at Pearl Harbor) and the work of some great people working together to try to understand what were the Japanese thinking, what were they going to do,” Rogers said.

Intelligence wasn’t the only reason for the U.S. victory.

The brave heroics by dive bomber pilots, Japanese mistakes and luck all played a role. But Nimitz himself observed that the code-breaking was critical to the outcome, said retired Rear Adm. Mac Showers, the last surviving member of the intelligence team that deciphered Japanese messages.

“His statement a few days later was ‘had it not been for the excellent intelligence that was provided, we would have read about the capture of Midway in the morning newspaper,’” Showers said in an interview.

Japan’s vessels outnumbered U.S. ships 4-to-1, Japan’s aviators had more experience and its Zero fighter planes could easily outmaneuver U.S. aircraft.

But Japan, unlike the U.S., had little knowledge of what its enemy was doing.

Japanese commanders believed a U.S. task force was far away in the Solomon Islands. Then, as June 4 neared and Nimitz prepared his troops, Japanese commanders failed to recognize signs of increased military activity around Hawaii as an indication the U.S. had uncovered their plans to attack Midway, the site of a small U.S. base.

The U.S. lost one carrier, 145 planes and 307 men. Japan lost four aircraft carriers, a heavy cruiser, 291 planes and 4,800 men, according to the U.S. Navy and to an account by former Japanese naval officers in “Midway: The Battle That Doomed Japan, the Japanese Navy’s Story.”

The defeat was so overwhelming that the Japanese navy kept the details a closely guarded secret and most Japanese never heard of the battle until after the war.

Nimitz got his intelligence from Showers and a few dozen others relentlessly analyzing Japanese code in the basement of a Pearl Harbor administrative building.

Japanese messages were written using 45,000 five-digit numbers representing phrases and words.

The cryptographers had to figure out what the numbers said without the aid of computers.

“In order to read the messages, we had to recover the meaning of each one of those code groups. The main story of our work was recovering code group meanings one by painful one,” Showers said.

At the time of the Dec. 7, 1941, attack on Pearl Harbor, they understood a small fraction of the messages. By May 1942, they could make educated guesses.