I'm going to start by recommending a very good and comprehensive book on this: At Dawn We Slept, by Gordon Prange, which goes into great detail as to why events played out as they did.

The summary version:

Roosevelt knew that war was coming, and the US military knew that an attack was going to happen somewhere. There were a number of signs that it would be Pearl Harbor, but these were misread. And, it was a surprise attack.

Now, to make sense of that summary:

By 1939, the Americans had broken the Japanese diplomatic codes. So, whenever a message was sent from Tokyo to a Japanese embassy or consulate on American soil, American intelligence was able to read it. These were called the "Magic" intercepts - they were top secret, and the list of those who needed to know was VERY small (and, in fact, did not include the Commander-in-Chief Pacific Fleet at Pearl Harbor).

So, as the Japanese government attempted to buy time through negotiations to get all of their proverbial ducks in a row for the invasion and conquest of the Philippines and Dutch East Indies, the American government knew from the decoded traffic that Tokyo was negotiating in bad faith. It also meant that the Americans knew in the days before the attack on Pearl Harbor that an attack was coming, and they were decoding, translating, and reading the 14-part message which would break off negotiations with the American government faster than the Japanese embassy was decoding it.

So, they knew that war was coming. Unfortunately, they had not broken the Japanese naval code yet, and the people who might have put the pieces together from the large number of conspicuous inquiries regarding the state and schedule of warships at Pearl either didn't see the messages in time or dropped the ball - they thought that the attack would be against MacArthur in the Philippines.

What made it a surprise attack was a combination of bad timing and the Japanese government really not caring about diplomatic niceties. The attack was scheduled to occur shortly after the 14 part message was delivered to the American government by the Japanese ambassador. Unfortunately, it was so secret that the ambassador was instructed to translate it himself, and as a result, he was over an hour late delivering the message...by which time the attack had begun.

By that measure alone, the attack would have been a surprise attack. However, the 14 part message was NOT a declaration of war - it was merely a breaking off of negotiations. So, even if it had been delivered on time, the attack would still be taking place against a nation Japan had not declared war against, and be a surprise attack.

There is a conspiracy theory that suggests that Roosevelt either knew about or manipulated the attack on Pearl so that the US could declare war on Germany, but it is just nonsense - not only would it mean that Roosevelt was starting a war with Japan by having Japan destroy the very instrument that would be needed to wage that war, but the US navy was already waging an undeclared war against German U-Boats in the Battle of the Atlantic - by itself, that would have escalated into a full war against Germany regardless.

If you want to see these events dramatized about as accurately as they can be, watch Tora Tora Tora (and avoid Michael Bay's Pearl Harbor, which is just...bad).

In short, that the US would be a shooting war with either or Japan and the European Axis by the end of 1941 was a surprise to basically nobody in the US government.

The US and Japan had spent 30 years planning how to fight each other as geopolitical tension around competing imperial ambition and spheres of influence became even worse with the Japanese invasion of China. The various iterations of War Plan ORANGE, were not just theoretical exorcises(and were remarkably accurate in the ebb and flow of the actual Pacific War), while other documents like the Plan DOG Memo show that senior US leadership felt that the US was going to find itself in a war soon and had to use that time to put itself in the best position when that happened.

The negotiations attempted in the Fall of 1941 were a last chance that as we now know the Japanese Govt didnt approach on the level. US bases were being reinforced across the entire Pacific, and the main fleet was settling in to its new home base in Hawaii a year after moving from California.

While in the Atlantic the US Navy and German U-Boats had already engaged in several battles under FDR's "Neutrality Patrols" with losses and dead on both sides. And the US Marine Corps had occupied Iceland to relieve its British garrison to fight elsewhere. It was not really in question that the US, by the second half of 1941, would continue to give more aid to the Allies, just what the timeline would be.

Now none of that of course involves FDR, or other US senior leaders, knowing the specifics of the coming attack by Japan prior to the morning of the 7th and the double blow of receiving reports from local commanders and the delayed 14-Part Message and deceleration of war. Intel sharing was imperfect, and often lacking important context, and even then analyzed to different conclusions. And War warnings were as much reminders to stick to existing heightened readiness plans as signals to be ready for something new. While on a practical level FDR was a President who loved his Navy, and it is hard to buy that he would have sacrificed the majority of its fighting strength and risk the the majority of its strength just to get Japan to attack first.