The 'sleeping giant' line is fiction, but the warning was real

Did a Japanese naval leader really say after Pearl Harbor that Japan had awakened a sleeping giant?

Not in that famous wording. The quotation is a Hollywood invention, although senior Japanese admirals had warned against the war before Pearl Harbor.

The famous claim has a Japanese naval leader declaring after Pearl Harbor that the attack had awakened a sleeping giant. The exact quotation is a Hollywood invention. The underlying caution was real, since senior Japanese admirals had warned well before the attack that war with the United States was a disastrous choice. Only a small number actively wanted such a war. More concluded that policy had left them no other option. The memorable fabricated line loosely represents documented strategic opposition.