The answer is in several parts.
At a political level we had an American president who just could not print enough money, quickly enough, to allow the mighty engine of American capitalism to overcome the inertia of a depression caused by tariffs and perpetuated by regulations and social programs. He needed the war to activate our industrial base and restore prosperity. His greatest “ally” was as tangled as tar baby with the interests of the empire that supported his island kingdom that was about to be cut off and forced to a negotiated peace that would have reduced it to third rate status fifty years before it actually happened.
At a military level you may call it the fog of war but a far more accurate description in that poor planning, poor surveillance, poor communications and a systemic inability to respond quickly and decisively hamstrung command in its efforts to prevent the attack, minimize the consequences of any attack that occurred and launch either pre-emptive or retaliatory strikes of our own. Costello does not give adequate weight to facts like MacArthur not even being an officer in the United States Army at the time of the attack – he had been retired from active duty and was field marshal [sic] of the Filipino Army.
Of course for the western democracies, where war is far too important to be left to the generals, we never seem to be adequately protected against foreign dictators or our own politicians. If you want to know why Pearl Harbor still matters so much look at the 70 years following and try to find 12 months where we were not actively engaged in either direct military intervention or clandestine operations to support allies or deter aggressors. I begin to believe that war is far too important to be left to the politicians.
Days of infamy : MacArthur, Roosevelt, Churchill, the shocking truth revealed : how their secret deals and strategic blunders caused disasters at Pearl Harbor and the Philippines John Costello New York : Pocket Books, c 1994 World War, 1939-1945 Diplomatic history Hardcover. 1st ed., and printing. xi, 448 p.,  p. of plates : ill. ; 25 cm. Includes bibliographical references (p. 424-434) and index. Clean, tight and strong binding with clean dust jacket. No highlighting, underlining or marginalia in text. VG/VG
Drawing on declassified American and British top-secret documents, John Costello reveals how major strategic and diplomatic miscalculations by Franklin D. Roosevelt and Winston S. Churchill together with the military blunders committed by General MacArthur set the stage for Japan’s successful attacks on Pearl Harbor and Clark Field. For the first time, he documents how it was the devastating loss of air power in the Philippines – and not the battleships lost at Pearl – which permitted Japan’s lightning conquest of the Far East in 1942.
For 50 years, Adm. Husband Kimmel and Gen. Walter Short have been blamed for the unpreparedness that led to the Japanese attack on Pearl Harbor. Costello reveals that the two Hawaii commanders were denied information that could have saved the Pacific Fleet battleships and the lives of thousands of U.S. servicemen. A far more heinous command failure, in his view, was that Gen. Douglas MacArthur allowed his air force in the Philippines to be destroyed on the ground 10 hours after the Pearl Harbor debacle; his refusal to launch a preemptive strike against Japanese airbases as ordered doomed the defense of the Philippines before it could begin. MacArthur’s inaction also contributed, the author contends, to the loss of Malaya and the Netherlands East Indies to the Japanese, because his bombers were the linchpin of a secret U.S. pact to defend British and Dutch territories in the Far East. Unlike Kimmel and Short, who had to retire in disgrace, MacArthur was never the subject of a formal inquiry. Although Costello clearly defines MacArthur’s mistakes, his treatment of “the secret deals and strategic blunders” of President Roosevelt and Prime Minister Churchill is less forthcoming.
Costello’s investigation into the advance warnings of Japan’s attack on Pearl Harbor rivals the investigation of the JFK murder for devising conspiracy theories to account for what happened. In its most sinister permutation, the “plot” has FDR deliberately withholding intelligence from the commanders in Hawaii so the country would go into war united. Actual responsibility for the disaster may be more prosaic and varied, involving bureaucratic turf wars, the substitution of the Philippines for Hawaii (at MacArthur’s behest, as a strategic bastion for B-17 bombers), and numerous intelligence lapses. The chronicle of the latter, according to Costello’s research, was contained in a 1946 navy report kept secret until 1993. Entitled “Pre-Pearl Harbor Japanese Naval Dispatches”, it concluded that the U.S. Navy had all the coded messages it needed to deduce the time and places of Japan’s commencement of hostilities–but they were not decrypted. This is not the last word on the run-up to the Pacific war, but these densely packed facts should arrest readers’ interest until the next secrets come out–said to concern Churchill’s advance knowledge of Japan’s war plans. Did the old bulldog conceal his information from the Americans? The British won’t tell us “that” .