Pearl Harbor certainly provided a reason. It was a massive assault on the United States even though Hawaii was not yet a state (it was a territory under the protection of the other 48)
I don't think FDR wanted war, but he was a powerful supporter of the American Effort once war was declared. I don't think there was a conspiracy, but you are not a conspiracy nut if you think so. There is some evidence for it. Personally, I don't believe he was anyone who wanted war. He was negotiating in good faith right up until the moment he knew that Pearl Harbor happened. Then he declared war without the consent of Congress. Japan had just stirred up a hornet's nest.
