To take out the Pacific Fleet at Pearl Harbor, Japan sent six of its frontline aircraft carriers across the Pacific in a risky amd unprecedented gambit on Nov. 26, 1941.

So much could go wrong.

The plan was so difficult and so dangerous that its initiator, Adm. Isoroku Yamamoto, commander of Japan’s Combined Fleet, acknowledged that “we must be prepared to risk complete annihilation.”

Most of the ships would require refueling along the way, and because Japan had a strategy of fighting in home waters, it hadn’t perfected the technique.

To ensure success, a massive “kido butai” mobile strike force was secretly assembled in northern Japan under the command of Vice Adm. Chuichi Nagumo.

Secrecy was key to achieving victory in Hawaii — and returning with as much of the fleet as possible to fight in the Southern Operation and British Malaya, the U.S.-ruled Philippines and the Netherlands East Indies.

The carriers Kaga, Akagi, Shokaku, Zuikaku, Hiryu and Soryu — the most flattops ever brought together — two battleships, two heavy cruisers, a light cruiser, nine destroyers, three submarines and eight supply ships were part of the Pearl Harbor task force.

Dozens of other submarines also were involved, including five that ferried midget subs for what would be an ill-fated harbor attack.

Cmdr. Minoru Genda, the air staff officer of the First Air Fleet, picked a route through the northern Pacific that provided concealment. The bulk of merchant ships operating between the United States or Canada and Japan or the Soviet Union sailed farther north, noted Gordon W. Prange, Donald M. Goldstein and Katherine V. Dillon in “At Dawn We Slept.”

The Second Submarine Squadron left Yokosuka, Japan, on Nov. 16 headed in a northeasterly direction scouting for the enemy. Around the same time, the carriers picked up their planes and crews. Other aircraft were flown into the bases to mask their departure.

Over several days in late November, Nagumo’s task force slipped into Hitokappu Bay in the northern Kuril Islands.

Rear Adm. Ryunosuke Kusaka, chief of staff of the First Air Fleet, outlined the plan for the voyage eastward: If the enemy sighted the task force before Dec. 7, Nagumo would turn his ships back to Japan. If the Americans fired on them prior to Dec. 7, the Japanese would slug it out.

Ships were to maintain radio silence — only to be broken when final orders were given for the attack. One by one, the ships left the harbor on Nov. 26, which was Nov. 25 in Hawaii.

The Americans had been keeping an eye on Japanese naval communications, and by mid-November, Cmdr. Joe Rochefort and the Combat Intelligence Unit at Pearl Harbor had some awareness of submarine movement.

“His reports showed a move eastward. And no one could miss the picture of a big Japanese buildup to the south,” the authors said in “At Dawn We Slept.” But the position of the carriers was entirely unclear.

Adm. Husband E. Kimmel, commander of the Pacific Fleet, asked about the carriers and was told that most often when they were not heard from, they were in port.

At night and during heavy overcast, Nagumo brought the ships closer together for easier communication — with signal flags and short-range blinkers, according to “At Dawn We Slept.” Some mornings he awoke to find one of his ships practically out of sight.

By Dec. 1, Nagumo’s task force had covered half the distance to Oahu. The same time frame brought awareness by the Americans that Japanese radio call signs had changed. Also ominous was a marked drop in radio traffic, indicating a possible emergency.

Lt. Cmdr. Edwin T. Layton, a Pacific Fleet intelligence officer, told Kimmel he could not locate two Japanese aircraft carrier divisions.

“Do you mean to say that they could be rounding Diamond Head and you wouldn’t know it?” Kimmel said.

“I hope they would be sighted before now,” Layton responded.