Relations had been deteriorating for decades between the United States and Japan prior to the attack on Hawaii.

By 1941 war was seen as inevitable. “War Plan Orange” for possible conflict with Japan had been updated through the 1920s and 1930s.

Meanwhile, Japan had for years war-gamed attacks on Pearl Harbor. America was certainly aware that Japan could attack the territory of Hawaii.

But would it? Most top U.S. military and government leaders scoffed at the idea, periodically drilling for it but thinking war would actually start elsewhere, such as in the Philippines.

Adm. Isoroku Yamamoto, commander in chief of Japan’s Combined Fleet, decided to bring that war to far-off Hawaii, against all odds, with the most powerful naval strike force ever amassed. At its core were six aircraft carriers.

Japan’s Naval General Staff had repeatedly considered attacking Pearl Harbor in war games.

“Each time the suggestion came up, however, the staff members dropped it, deciding it was impossible,” said Gordon W. Prange, Donald M. Goldstein and Katherine V. Dillon in “At Dawn We Slept.”

The 3,400-mile steaming distance for the Japanese fleet was too great. The risk to the ships was too high. Pearl Harbor’s shallow draft of 40 feet would prevent a torpedo attack. Hawaii had been built up with ships and air power — because of Japan’s increasing aggression in Asia — presenting a formidable defense.

The Harvard-educated Yamamoto prepared for every possible contingency, though, and in 1941 Japan felt boxed in. Japan had invaded North China in 1937.

In 1939 it took China’s southern Hainan island.

Now it set its sights on Southeast Asia and Malaya, the Philippines and Dutch East Indies.

“The Japanese convinced themselves that necessity and self-protection demanded that they take over the vast resources of promised lands to break through real or imagined encirclement and beat off the challenge of any one or a combination of their international rivals — the United States, Great Britain and Soviet Russia,” the authors of “At Dawn We Slept” wrote.

By the end of 1940 the United States, seeking to aid China, embargoed the export to Japan of all war materials except petroleum. In the spring of 1940 a large portion of the U.S. Navy fleet was moved to Pearl Harbor.

In December 1940, Yamamoto wrote, “The probability is great that the launching of our operation against the Netherlands Indies will lead to an early commencement of war with America.” Yamamoto felt Japan could seize the initiative and strike decisively at Pearl Harbor.

Japanese war strategy was to lure the enemy to its home waters so attack would benefit from ready resupply. Yamamoto upended that with the Pearl Harbor attack plan. With a background in naval aviation and realizing its potential when battleships were still king, Yamamoto envisioned a task force of aircraft carriers, cruisers and destroyers.

With a surprise strike on the U.S. fleet in Hawaii, Yamamoto hoped to protect Japan’s southern flank in its Southeast Asia campaign and gain at least a six-month window of advantage over an angered America.

“If Japan could move fast enough and hard enough in the breathing spell thus gained, it might conquer those vast regions, thus securing the resources it so urgently needed to carry on a protracted war,” according to “At Dawn We Slept.” “It might also consolidate its position to the point where a negotiated peace acknowledging the status quo might be possible.”

In October 1941 Yamamoto still faced plenty of opposition within the Japanese navy to his risky Pearl Harbor plan — going so far as threatening to resign with his entire staff if it wasn’t adopted.

In Hawaii, Adm. Husband E. Kimmel, commander of the U.S. Pacific Fleet, thought a submarine attack would be the most feasible and probable Japanese course of attack.

In 1939 the Japanese had experimented with wooden fins attached to torpedoes acting as stabilizers. By summer of 1941 refinements resulted in 70 percent of the undersea weapons running at 40 feet when launched at less than 100 feet at air speeds below 150 knots. Advancements also were made in aerial bombing.

One of the most daring naval operations of all time was on.

On Nov. 23, 1941, aboard the carrier Akagi, Cmdr. Minoru Genda, the First Air Fleet air staff officer, said the “primary objective of the attack is to destroy all U.S. carriers and at least four battleships.” Closely related was the “annihilation of U.S. air power on Oahu.”

The Malay attack force departed Japan on Nov. 24. Two days later the Pearl Harbor group left from the northern Kuril Islands. The southern Philippine invasion force pulled out the same day.

In late November in Hawaii, Kimmel, Lt. Gen. Walter C. Short, commander of the Army’s Hawaiian Department, and other key personnel discussed a proposal to send P-40 fighters to Wake and Midway islands, the authors of “At Dawn We Slept” wrote.

Lt. Col. James A. Mollison, chief of staff of the Hawaiian Air Force, registered disapproval.

“Why are you so worried about this?” Kimmel asked. “Do you think we’re in danger of attack?”

“The Japanese have such a capability,” Mollison replied.

“Capability, yes, but possibility?” Kimmel said, asking Capt. Charles E. McMorris, the Pacific Fleet war plans officer, “What do you think about the prospects of a Japanese air attack?”

“None. Absolutely none,” McMorris responded.