Who is quoted as fearing that their actions awakened a vengeful giant?

The correct answer is Admiral Yamamoto. This quote reflects his belief that Japan's attack on Pearl Harbor would provoke the United States, leading to severe consequences for Japan. Admiral Isoroku Yamamoto, who was the commander-in-chief of the Imperial Japanese Navy, anticipated that the attack would awaken a powerful and determined response from the American military, which would likely mobilize substantial resources and manpower against Japan. His concerns were rooted in an understanding of America's industrial capacity and commitment to military engagement, which he feared would overwhelm Japan in the long term.

In contrast, other figures mentioned, such as Admiral Nimitz, General Tojo, and President Roosevelt, do not have documented quotes that convey a similar sense of foreboding regarding the awakening of a "vengeful giant." Nimitz and Roosevelt, for instance, were more focused on the immediate response and strategy following the attack, while Tojo, as the Prime Minister of Japan during World War II, was more involved in the aggressive expansionist policies that led to conflict rather than expressing concern about potential repercussions of their actions.