The reality was a bit more complex than just arrogance, though there was plenty of that still in the mix. The American high command DID believe the Japanese were going to attack, and they DID send reinforcements to the locations they thought the Japanese would hit. However, they became fixated on what they thought was the most logical target and believed Pearl Harbor wasn't on the hit list. They REALLY believed that the Panama Canal was the real target because if you shut that down with heavy bombing, it would take years to fix and would effectively cut the American fleet in half by preventing and reinforcements from being sent from the Atlantic easily. The American military there was reinforced with extra troops, lots of anti-aircraft guns and placed on high alert.The American were so arrogant that they did not think it was possible for the Japanese to attack Pearl Harbour
Ironically, of all the locations the American thought would be attacked (the Philippines, Guam, and the Dutch East Indies) were attacked EXCAPT the Panama Canal. The general idea was that Pearl Harbor wasn't on the list because it was too large and heavily defended a target to be risked by an attack force so far away from the attacker's home waters.
EDIT: What the Americans feared at Pearl was sabotage, either by local Japanese immigrants still loyal to the homeland or by commando teams smuggled in via Japanese submarines. The former turned out to be just a racist pipe-dream the the latter appeared possible because the American navy had intercepted a couple of Japanese subs about a month earlier.
The grand irony that played out was the reason the Japanese had such an easy time of bombing the planes and ships at Pearl Harbor was that they had been set up in formations to help patrols spot potential saboteurs by putting each ship in a double line where the patrols on each ship could keep an eye on 3 other vessels while they walked their own ship and the planes were lined up to allow sentry patrols to easily see if anyone was trying to sneak around the airfield. Tat was all well and good for preventing sabotage, but made for absolutely excellent targets for bombers.
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