Japan decided to strike before the United States fleet expanded
For Japan, a vastly expanded US Navy in the Pacific was unacceptable. A new and highly militaristic government seized power in Japan, and plans for the expansion of the empire into Southeast Asia and the South Pacific accelerated. Rather than deterring the Japanese, the placing of the American fleet at Pearl Harbor reinforced the need, in their minds, to eliminate it from the board. Japanese military planners were well aware the British could not concentrate too large a naval force in the Pacific and Indian Oceans. The Royal Australian and New Zealand Navies were of little consequence, as were the fleet units of the Dutch, defending the Dutch colonies despite the occupation of Netherlands by the Germans. Only the US Navy presented a deterrence to Japanese dominance, and the Japanese rightly considered the idea America did not want to go to war with Japan, or with anyone else.
America’s attention was drawn to the war in Europe, especially to the stirring radio broadcasts of the new British Prime Minister, Winston Churchill, who took office following the collapse of the French. Churchill painted vivid word pictures of the horrors of war at the hands of the Nazis, invoking propaganda images of the brutal Hun of the First World War. By contrast, to most Americans the Japanese were characters out of Madame Butterfly, residing in quaint paper houses with charming gardens filled with cherry blossoms and chrysanthemums. Gradually the reports of atrocities committed by the Japanese in China changed that image, at least for those paying attention. Most Americans did not. As 1941 entered its second half, Americans enjoyed newly-found prosperity, with manufacturing booming as Lend-Lease to Britain and the Soviet Union stimulated the economy. War with Japan was unthinkable.