America's optimism was an understatement... "everyone knew," we would win. Sure, there would be set backs from time to time but, the ultimate victory was assured, in America's opinion. Remember, the war in the Pacific was nationalistic as well as racial! Americans felt it was better than Japan because Japan was an Asian nation. The Japanese felt they could win because of their racial purity, they were the best of Asians! Plus Asians were better than non-Asians, especially a mongrel nation such as the USA!
OPTIMISM??? in January of 1942 President Roosevelt ordered a committee to start the planing of what will be required to invade the Japanese home islands! That is why each island in the Pacific captured, was turned into a storage facility! The invasion of Japan would require a mass supply train. The American people at first wondered how long and how much time it would take to defeat the Japanese in particular and the Germans. The first string of victories by the Japanese showed this war would be difficult but, by June the USA had some victories. The Doolittle raid, the Coral Sea and, Midway Island... we had turned the corner and the USA was on its way.
A good example of the Japanese arrogance is, the USA told the USSR that we had broken some Japanese codes. A German spy in the USSR got that information back to Berlin, which informed Tokyo. The Japanese attitude was, "Occidentals have a hard enough time learning to speak Japanese, how are they going to understand Japanese that is coded? (How about having a cryptographer work with a translator??)
The Japanese should have known their codes had been broken. American actions at Midway, Guadalcanal, the assassination of Adm. Yamamoto, etc. should caused all sorts of alarm bells to go off?
US Press Screw-Ups URL;
http://www.strategypage.com/htmw/htiw/articles/20061201.aspx
The full and interesting story of the Chicago Tribune blowing the codebreakers is at
http://www.newseum.org/warstories/essay/secrecy.htm. The actual headline was "NAVY HAD WORD OF JAP PLAN TO STRIKE AT SEA."