With the attack on the US Pacific Fleet on December 7, 1941, Japan had brought the most powerful nation on earth into the war on the side of the Allies.
Japan's war planners, specifically Isoroku Yamamoto, never gave any thought the galvanizing effect a surprise attack might have on the American public. Since the war it has been a source of debate among historians what would have happened if Nomura and Kurusu had delivered the declration of war on time. It is unlikely a timely delivery would have much effect in preventing the United States from crying out for blood with only a half-hour warning. Curiously the psychological effect of such an attack was never considered.
The Pearl Harbor attack was just part of an orchestrated, Pacific-wide assault. Seriously underestimating the naval, air, and ground strength of Japan, the Allies assumed that they would be able to ward off blows that would come in succession. They did not expect the Japanese to attack all predicted targets at the same time. As casualties and losses mounted, it would have been unlikely that the United States would have responded any other way than total war.
Roosevelt, declaring war on December 8, 1941, declared the previous day a “date that will live in infamy,” and listed all of the places in the Pacific that the Japanese attacked. Still, the serious condition of the US Pacific Fleet was kept a secret, partly out of the desire to not panic the public, especially the west coast. Also, the War Department did not wish to give information to the enemy, which they assumed was reading American papers. A serious crisis had befallen the Americans.