Perhaps one of the longest and most enduring conspiracy theories in this country is the belief that the Pearl Harbor attack on December 7, 1941 was known before hand, and was allowed to happen because Franklin Roosevelt wanted to go to war with Germany and Japan.
Besides the fact that there is no proof that FDR actually knew the attack was actually going to happen, he actually didn't want to go to war with anyone, but was very concerned with Germany and knew that the country may need to go to war with that country, which is why there was an arms build up in this country before we entered the war, which was supported by the Democrats and the Republicans, and why there was a "Europe First" battle plan, because Europe was considered more critical then the Pacific was.
Many people believe that plans for the attack were intercepted by the intelligence community and intentionally ignored. You have to understand that our intelligence agencies weren't very good back then, and often times did not communicate with each other. The reports were not ignored, they mishandled them, and nothing really gave a clear picture about what the Japanese military was planning, and so no one had any real reason to suspect they were going to attack Pearl Harbor, at least in the way they did.
One of the myths about this is that the planes were spotted on radar and intentionally ignored by the base commander, Admiral Husband E. Kimmel. Yes, it is indeed true that the planes were picked up on radar, but they were thought to be US bombers that were coming in from the mainland, that were scheduled to come in that morning, and in fact did land during the attack itself. Also, one of the myths about this is that Adm. Kimmel was made aware of the incoming planes, and ignored them, but, this is not true. Adm. Kimmel didn't ignore them, he wasn't made aware of the planes coming in. It was a lieutenant who was made aware of the incoming planes, and believing they were the bombers that were suppose to come in, didn't bother to tell Adm. Kimmel, because it was most likely not important enough to bother him with.
Many people also cite the fact that none of the three carriers that were stationed there were out at sea at the time of the attacks as proof that the Navy knew an attack was going to happen, and that the Navy sent them out in order to save them. The problem with is that carriers back then weren't considered the primary ships of the fleet as they are today. Battleships were the backbone and work horses of the fleet back then, and at the time, were considered more valuable then carriers, which had been untested in battle. Also, the Navy only had six carriers before we entered the war, and we had a lot more battleships, so it isn't surprising that some of them were in port at the time.
Besides the fact that the carriers were not the work horse of the fleet at that time in history, if the US Navy did know that an attack was coming, they would have most likely moved their ships out of the harbor and into open sea, rather then just let them sit around and get damaged or sunk, which not only weakened the Navy's power due to the lose of the ships, but also clogged up the ports with the damaged and destroyed ships. In fact two of those ship, the USS Utah and the USS Arizona, are still there, under the water, because they were to heavily damaged to move.
The fact is that even after over 70 years, there has never been any real proof what so ever that the US government knew ahead of time that Japan was going to attack Pearl Harbor. When you look at the evidence, this conspiracy theory is very logically faulty.