Japanese attack China

In 1931, the Japanese army invaded the northern Chinese province of Manchuria. Japan ignored American protests and, in the summer of 1937, launched an attack on the rest of China.

Although alarmed, no nation (including America) wanted to use military force against Japan. Instead, President Roosevelt imposed sanctions. Relations soured further.
Japan demanded the United States lift its embargo – a situation that was unacceptable.

In May 1940, in a further attempt to deter Japanese expansion, Roosevelt moved the United States fleet to Pearl Harbor with Husband E. Kimmel as commander-in-chief.
Both sides refused to back down and relations reached boiling point. Many in the Japanese high command saw war with America as inevitable; including Admiral Isoroku Yamamoto, commander-in-chief of Japan’s combined fleet.

Yamamoto had lived in America as Naval attaché in Washington and also studied at Harvard University in the 1920s. As the leader of any potential attack, he reasoned that Japan could not win a long war against America’s superior economic and industrial power. He believed their only chance of success was to achieve a quick and decisive victory.

As Stephen Ambrose explains in Legacy of Attack: "[Yamamoto] said, I don’t care if we march troops down Pennsylvania Avenue, we’re not going to conquer the United States…if we’re going to have any chance of winning this war, we’ve got to destroy the American fleet, and that’ll give us six months to run wild in the South Pacific. We can build a defensive barrier that will be very difficult for the Americans to crack. And at some point, they’re going to say, ‘We quit. Keep your gains.’ " And so the Japanese began to plan their attack.