There were plenty of indicators that Japan could attack Hawaii and Pearl Harbor.

But in 1941 many in the American military, government and public didn’t want to believe that it would happen.

U.S.-Japanese relations had often been strained since the early 1900s. Amid Japan’s growing military might and expansionist aspirations, its gains in the Shandong region of China after World War I caused U.S. diplomat E.T. Williams to opine, “They (the Japanese) are after the conquest of Asia, as a preliminary to world conquest.”

Even then the Imperial Japanese Navy was the third largest in the world behind Great Britain and the United States. America was a potential enemy.

The possibility of conflict with Japan resulted in the United States developing a new war plan in 1924. Hawaii took on a greater military role to deter Japan, and by 1938, according to the U.S. Army Center of Military History, $75 million had been spent on Pearl Harbor, with the Army forking over twice that on military installations to protect the fleet.

Naval expert Hector Bywater wrote “The Great Pacific War” in 1925, envisioning a great naval conflict between Japan and the United States that included Japan’s invasion of the Philippines and Guam, and an island-hopping campaign to get them back.

No Pearl Harbor attack was foreseen, but Bywater described insurrection on Oahu by Japanese residents who cut telephone lines, took up smuggled arms against U.S. forces and were eventually routed with the help of an arriving tank unit.

In 1932, during annual Army and Navy exercises, U.S. Navy Rear Adm. Harry Yarnell led the aircraft carriers Saratoga and Lexington on a mock attack on Oahu from the northeast and under radio silence. More than 150 biplanes took Oahu’s airfields and Pearl Harbor by surprise, on a Sunday morning, landing a knockout blow to U.S. defenses.

Five years later another exercise again showed the effectiveness of carrier air power — and Hawaii’s vulnerability to it.

The Saratoga sailed from San Diego and “launched a surprise air attack on Pearl Harbor from a point 100 miles off Oahu, setting a pattern that the Japanese copied in December 1941,” Navy history notes.

In January 1941, several months after Rear Adm. Patrick N.L. Bellinger became commander of Hawaii’s Patrol Wing 2 and its reconnaissance planes, he wrote to the chief of naval operations to say that he had arrived “with the point of view that the international situation was critical, especially in the Pacific.”

Adm. Husband E. Kimmel, commander of the U.S. Pacific Fleet, assigned Bellinger and Maj. Gen. Frederick L. Martin, commander of the Hawaiian Air Force, to draft a plan for an attack scenario on Oahu.

In the March 31, 1941, report, the “authors virtually foretold the future,” said Gordon W. Prange, Donald M. Goldstein and Katherine V. Dillon in “At Dawn We Slept.”

A declaration of war might be preceded by a surprise Japanese attack.

“It appears that the most likely and dangerous form of attack on Oahu would be an air attack,” the officers said. “It is believed that at present such an attack would most likely be launched from one or more carriers which would probably approach inside of 300 miles.”

The Japanese navy felt the attack was feasible given the greater capabilities of newer aircraft and modifications to aerial torpedoes, according to the U.S. Navy. Aircraft carriers and naval aviation would be used on an unprecedented scale.

Lots of Americans, however, still didn’t believe such an attack was even remotely possible. That included Martin, who had nailed the Japanese attack plan.

He later told the Army Pearl Harbor board there would be no Japanese strike “because if it failed, it meant such a reduction in their striking power they would be confined to their home islands from then on,” the authors of “At Dawn We Slept” wrote.

Thurston Clarke, in “Pearl Harbor Ghosts,” said the American overconfidence bordered on arrogance or delusion.

Clarke noted that in 1940 Maj. Gen. Charles Herron, then commander of the Hawaiian Department, announced, “Oahu will never be exposed to blitzkrieg attack. This is why: We are more than 2,000 miles away from land whichever way you look, which is a long way for an enemy to steam. And besides, it would have to smash through our navy.”

Clarke also referred to a Sept. 6, 1941, Honolulu Star-Bulletin article in which journalist Clark Beach wrote, “A Japanese attack on Hawaii is regarded as the most unlikely thing in the world, with one chance in a million of being successful.”