The US administration was definitely expecting conflict. 10 days before before Pearl Harbor they alerted all Pacific forces to the breakdown in diplomatic relations:
This dispatch is to be considered a war warning X Negotiations with Japan looking toward the stabilization of conditions in the Pacific have ceased and an aggressive move by Japan is expected within the next few days X The number and equipment of Japanese troops and the organization of naval task forces indicates an amphibious expedition against either the Philippines Thai or Kra Peninsula or possibly Borneo X Execute an appropriate defensive deployment
Similarly in the Atlantic, Roosevelt was intentionally escalating tensions. In September 1941, he publicly declared German / Italian warships unwelcome in US and US supported waters of the North Atlantic. He had given this order privately earlier. Not very neutral. Roosevelt addressing the country:
Let this warning be clear. From now on, if German or Italian vessels enter the waters, the protection of which is necessary for American defense, they do so at their own peril… When you see a rattlesnake poised to strike, you do not wait until he has struck before you crush him. These Nazi submarines and raiders are the rattlesnakes of the Atlantic.
Also, the the US defense budget in 1941 increased 383% over 1940. The draft was signed into law September 1940. The first concept for the B-29 bomber was submitted in 1940. The USS Essex, the first of the new fleet carriers critical to victory in the Pacific started construction April 1941.
So war preparation was quite underway at the time of declaration of war. The exact method / timing of entry into the war was a surprise. But most people could see the diplomatic trends coming.
https://www.smithsonianmag.com/history/how-almost-everyone-failed-prepare-pearl-harbor-1-180961144/
https://www.cfr.org/blog/twe-remembers-fdrs-shoot-sight-fireside-chat