It changed the course of history. Up until Pearl Harbor, isolationists in America had succeeded in keeping the US out of WWII, despite the unanimous opinion of the American establishment. In December 1941 the Nazis were well on their way to conquering the world. They had invaded and occupied Norway Denmark, Holland, Poland, Belgium, and France. Britain was on the ropes, they had fended off the Nazi air attack in the summer of 1940 by the skin of their teeth, but were in no shape to do much more. The vast Red army, locked in combat with the Wehrmacht, had suffered defeat after defeat, loosing hundreds of thousands of men in German encirclements. By Pearl Harbor time the Germans had reached the suburbs of Moscow. Had Moscow fallen, Russian resistance would have collapsed and Adolf Hitler would rule all of Europe from the Channel to the Urals. Had the isolationists kept America out of the war for another year or two, Hitler might have won. It was a close run thing.
Isolationism disappeared in the smoke of Pearl Harbor. Americans were outraged and to a man demanded their government do something about it. Which the Roosevelt administration pr0ceeded to do.
The Japanese, with the exception of Admiral Yamamoto, totally misread the situation and
American intentions. The Japanese war aim was to conquer China, plus a few other things, but China mostly. The Japanese economy was dependent upon American exports of gasoline and crude oil and scrap metal. The Americans disapproved of the China invasion and embargoed those crucial exports. The Japanese were faced with collapse of their economy (production of warships, war material, aircraft and all the rest needed to maintain a war), or backing off, with the intolerable loss of face that would entail. They never thought about going elsewhere for raw materials. Sumatra, not far away, had enough high quality crude oil production to run Japan thruout WWII. They could have just muscled their way into Sumatra, acquired the needed oil. The Americans would send diplomatic nastygrams to Tokyo, but the US isolationists would not have permitted anything more.
Instead, Japan thought that a devastating attack, one that knocked out the US fleet, would cow the Americans into making terms. Partly this mistake came from a Japanese leadership had no conception of the resources at America's disposal. In Japan, things were so tight that building a single new battleship required contributions from school children (lunch money) and years of scrimping and struggle. In America Roosevelt could pick up the phone and say " We need ten new battleships as soon as possible. The contract will be cost plus. Start work now". And ten new battleships, plus carriers, destroyers, liberty ships, submarines, and everything else would slide down the launching ways and join the US fleet. Japanese leadership simply did not understand this. They thought that sinking all the Pacific Fleet battleships would cripple the Americans forever.