The conspiracy theory (I have heard ones that Churchill knew because the British had broken the German codes and the Japanese invasion was mentioned.......among other thing, the only problem with that is the British only had German navy messages, and they only broke a percentage of them, they never broke the German diplomatic codes). Another theory which has some sprinkling of truth to it was the US had broken the Japanese codes and knew it was coming, problem was as of December, 1941 they hadn't broken any significant codes. Yeah, I have heard the arguments about the carriers being at sea, how that proved they knew, but they easily could have come up with an excuse to move the regular fleet out without tipping the japanese off, likewise they could have had the planes and their crews ready, instead of sitting wingtip to wingtip ready to be destroyed. The coventry argument doesn't hold (where the brits knew coventry was going to be hit with V2's but couldn't do anything), the navy had choices had they really believed an attack was coming.
I think most people knew war was inevitable but the military is not different then any other large organization, there is a lot of disbelief there, a lot of 'the US is not at war we are neutral, there is no reason to attack us", as well as more then a bit of racism, who are a bunch of short, bandy legged guys wearing coke bottle glasses going to take on the US? (and yes, those views were expressed, which is kind of ironic, given that by December, 1941 our military was still very, very weak).
More importantly, if they knew they could have protected the fleet and then had their excuse when the Japanese did attack, to lose a good part of our Pacific fleet when there would have been war anyway doesn't make any sense..and despite a cottage industry of books and such, no one has ever presented evidence that backs this up, it is typical conspiracy theory stuff, great for selling books. It doesn't surprise me this is coming a lot of the time from the nutty right wing, who almost 70 years after the man's death want to beat up on FDR and find any dirt they can levy....it also might be an attempt to divert attention from another fact, that the prime movement to keep the US out of the second world war , the America First movement, was mostly made up of middle america conservatives, like Vandendberg and Taft (and several conservative politicians, even after Pearl Harbor, declared that the US should not get involved in WWII, saying pearl harbor was a mistake or that the administration "baited" the japanese into attacking pearl harbor because he wanted the US in WWII......to their credit, after Pearl harbor the America First mentality was disavowed by most, especially after its pro Nazi underpinnings came to light.
I cannot disagree with any of this, other than the bit about conservatives. It's true that I do not like Roosevelt (I'm libertarian), but I think that allowing the Pearl Harbor attacks to occur would have been a perfectly acceptable action, given the possible alternatives. As much as I hate Roosevelt's domestic policy, most of his wartime policies were very good.