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.
Shoudl they begin to pull back the buildup on the Atlantic? Roosevelt, who wanted war with Germany, particularly worried that the European war, a far greater threat in his estimation, would be seen in the United States as a European problem and Japan as the Americans sole concern. Within days, Hitler would alleviate his concerns.
In Berlin on December 8, 1941, Adolf Hitler was elated. We have ally that has not been defeated in 1500 years! he told Minister of Propaganda Josef Goebbels. On December 11, 1941, Hitler gave a speech to the Reichstag. Confused and rambling, he compared his own childhood of poverty to that of the wealthy Roosevelt. He declared war on the United States.
In doing so, he ensured his own destruction. UK Prime Minster Winston Churchill, when he heard of Pearl Harbor, remarked, so we have won after all! The American public would have been quite content with dealing with Japan and leaving the European War to the Europeans. The treachery of the Japanese attack burned bright in the minds of most Americans, and they wanted revenge. If not for the declaration of war by Germany, Roosevelt would have had a hard time justifying declaring war on Germany until Japan was destroyed. But Germany did declare war, and the U-boats moved the Eastern seaboard in January 1942.