In January 1941, Adm. Isoroku Yamamoto, commander of Japan’s Combined Fleet, used brush, ink, rice paper, and the rich flourish of calligraphy to compose a letter to Navy Minister Koshiro Oikawa, marked “to be burned without showing to anyone else.” Yamamoto was putting into writing a scheme that, until now, had circulated among Japanese officers only by word of mouth. War was inevitable, Yamamoto said. He wanted to attack Pearl Harbor. No man had better reason to know his enemy. Yamamoto had studied English at Harvard (1919-1920) and had been naval attaché in Washington (1926-28). He had traveled the U. S. heartland. Fearful that Japan could never match America’s industrial potential, the admiral wanted to smash U.S. morale, then secure quick victory by surprising the U.S. fleet and sinking its aircraft carriers, and battleships at their moorings.

He wanted to attack Pearl Harbor.

Yamamoto was proposing a dramatic change of Japanese strategy, which until then had been focused on luring the U.S. fleet into a set-piece naval engagement in the southwest Pacific. Yamamoto’s proposal eventually reached a relatively junior officer, Cmdr. Minoru Genda, air staff officer on the carrier Kaga, who believed an attack could succeed but saw two snags. Genda warned that aerial torpedoes dropped into the shallow waters of Pearl Harbor (typically, 30 to 45 feet deep) would dig into the bottom before they could run their courses. More importantly, Genda cautioned that a Pearl Harbor attack plan would certainly fail if Japan did not achieve surprise.

Genda, of course, had no clue that Americans were reading Japanese mail. In 1940, U.S. War Department cryptanalysts had cracked the Japanese cipher code, known as “Purple.” A select few U.S. officials began reading the code intercepts, dubbed “Magic.” President Franklin D. Roosevelt and other U.S. leaders were thus well aware of Japan’s diplomatic maneuverings, although not of naval movements.

And the U.S. commander at Pearl Harbor, Adm. Husband E. Kimmel? In October 1941, Rear Adm. Kelly Turner assured the chief of naval operations, Adm. Harold “Betty” Stark, that Kimmel knew of the Magic intercepts and was reading them regularly. Turner, described by historian Gordon Prange as “a brilliant but corrosive man of enormous ego and ambition,” was wrong. U.S. officers in Hawaii, Kimmel included, did not have access to Magic.

 

Aircraft Carriers

For Japan, the key to success was the aircraft carrier.

For Japan, the key to success was the aircraft carrier. No other nation had started earlier or worked harder to develop these new capital ships. Yamamoto, Genda, and others would never have planned the Pearl Harbor attack had Japan not been a world leader in the new weapon of naval warfare.

In the 1920s and 1930s, while world naval conferences haggled over the size and shape of battleships, Japan pioneered the aircraft carrier. The Hosho, a 7,470-ton aircraft carrier built at Tsurumi, Japan, was completed in December 1922, and may have been the first ship in the world built as an aircraft carrier from the keel up. During much of the 1920s Hosho was Japan’s only flattop and, like her U.S. Navy contemporary, the converted collier USS Langley (CV 1), was developed for carrier operational techniques and tactical doctrine. Though superseded by far more capable ships during the later 1920s, she continued in operation well into World War II.

By 1941, Japan had more and newer carriers and carrier aircraft, including the incomparable Zero fighter. The U.S. was also a leader in this new field of naval warfare, but American carriers would not be a factor at Pearl Harbor.

By 1941, Japan had more and newer carriers and carrier aircraft, including the incomparable Zero fighter.

Japan genuinely hoped no war with the U.S. would be necessary, all the way into late 1941. Throughout 1940 and 1941, Japan sought a diplomatic solution that would persuade the U.S. to accept its dominant role in Asia, including its occupation of much of China. But on July 25, 1940, Roosevelt signed a proclamation ending the sale to Japan of all petroleum, petroleum products, and scrap metals. Although this was quickly watered down to become merely an embargo on aviation gas and certain metals, some in Tokyo took it as a signal that war was inevitable.