"US defense officials became fixated with the notion that Japan could be deterred by a show of force. This became especially true after the Joint Board approved the plan to dispatch several squadrons of medium-range B-17 bombers to the Philippines. The decision to bolster American strengths in the Far East was made during summer 1941, out of concern that Japan's alignment with the Axis powers necessitated protective measures. In October, Secretary of War Henry Stimson told Roosevelt that the bomber threat "bids fair to stop Japan's march to the south." By reinforcing the Philippines, the Americans could bide their time, and avert hostilities until their military position was strengthened. Large sections of the military leadership also concluded that Allied forces could inflict crippling losses on a Japanese expedition. The belief was based on an underestimation of Japan's military strengths and its determination to eliminate Allied positions in Southeast Asia. The War Department's planning division contended that the Associated Powers should attempt to halt Japan along the "general line of Hong-Kong to the Philippines," the latter of which held the key to maintaining the line. South of this line were "successive positions from which the combined ground, air and naval forces of the Associated Powers could exact a tremendous toll."
The United States thus did not anticipate a war against Japan, and the attack on Pearl Harbor was a genuine surprise. Conspiracy theorists have claimed that President Roosevelt knew about the Japanese plan, but kept the information secret from the public in order to allow the Imperial forces to carry out their operations and thereby give the US a valid pretext for declaring war. However, the theories are based on tenuous evidence. A more credible argument is that the US intelligence community had only vague data on what Japan intended to do. The government and military leadership continued to keep their policies under a tight veil of secrecy. Under the circumstances, the Americans could receive only hazy indications of their adversary's intentions. For example, in March, the naval attaché in Tokyo quoted a statement by a former Japanese admiral to the effect that war against the US would commence with the navy conducting attacks against the Philippines and Hawaii. However, the statement was made long before the navy had finalized its strategy, and appeared more as a boastful announcement.
Between December 1941 and early 1942, while Japan made its lightning conquest of Southeast Asia and the western Pacific, its navy and army appeared invincible to the Allies. Indeed, the Japanese victories owed themselves largely to skilful planning, along with the tactical and technological efficiency of their armed forces. The weak state of the US and British Empire also played an important part in facilitating Japan's successes. Yet, as early as March 1942, the high command had to contend with many of the weaknesses which plagued its war machine, the most important of which was that neither the IJN nor IJA had the capacity to defeat the Western powers in a prolonged conflict. The Imperial forces were overstretched, and America had not been knocked out of the war. On the contrary, the US was preparing to strike back, and most importantly, it possessed the industrial resources to build a military force that was far superior to anything the Japanese could deploy. Yet the military leadership failed to comprehend the predicament it faced, and maintained that Japan could deal a crippling blow on its opponents and thereafter secure its conquests against enemy invasions. The misperception led the Japanese to embark on a number of failed ventures in the Indian Ocean and Pacific areas which eventually culminated with the IJN's defeat at Midway in June 1942. The latter encounter was arguably the single battle which turned the tide of the war in the Allies' favor, and emasculated Japan's capacity to conduct further territorial conquests…"
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