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, 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 Friday.

Intelligence wasn’t the only reason for 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,”’ said 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.

A key breakthrough came when they determined Japan was using the letters “AF” to refer to Midway.

Showers said Cmdr. Joseph Rochefort, the team’s leader, and Nimitz were confident the letters referred to the atoll. But Adm. Ernest King, the Navy’s top commander, wanted to be sure before he allowed Nimitz to send the precious few U.S. aircraft carriers out to battle.

So Nimitz had the patrol base at Midway send a message to Oahu saying the island’s distillation plant was down, and it urgently needed fresh water. Soon after, both an intelligence team in Australia and Rochefort’s unit picked up a Japanese message saying “AF” had a water shortage.

Showers was an ensign in the office, having just joined the Navy. He analyzed code deciphered by cryptographers, plotted ships on maps of the Pacific, and filed information.

Now 92 and living in Arlington, Va., the Iowa City, Iowa, native went on to a career in intelligence. He served on Nimitz’s staff on Guam toward the end of the war, and returned later to Pearl Harbor for stints leading the Pacific Fleet’s intelligence effort. After the Navy, he worked for the Central Intelligence Agency.

Showers said commanders weren’t always as open to using intelligence to plan their course of attack the way Nimitz was. Some were suspicious of it.

But Midway changed that.

“It used to be a lot of people thought intelligence was something mysterious and they didn’t believe in it and they didn’t have to pay attention to it. Admiral Nimitz was fortunately what we call intelligence-friendly,” Showers said.