FDR goaded the Japanese into an attack and they stupidly complied
The United States of America was attacked at pearl harbor December 7th 1941 by a far outmoded government Administering a far outmoded regime and set of beliefs and culture, that had recently been newly industrialized by the UK, and then trade with the USA...
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If FDR had not taken a stand against Japan's fascistic aggression it would have been a matter of time until the entire Far East would be under Japanese control.
At that time, Hitler Germany, which was allied with Japan, was preparing to conquer and occupy England, which its forthcoming V-2 rocket program was certain to facilitate.
At the same time, Mussolini's fascist Italy was aligning with Hitler's intention to sweep across and occupy the entire European continent, which, without aid from an industrially powerful nation (the U.S.), was as certain as was Japan's inevitable conquest of the Far East and, eventually, Australia.
At the same time, Germany was making inroads onto oil-rich Northern Africa, which could easily have supplied the energy needs of an emerging unified fascist empire.
If the combined ambitions of Hirohito, Hitler and Mussolini had resulted in a fully accomplished, irresistibly powerful allied war machine, which would have taken no more than two or three years, there is no way the North-American continent could have withstood a unified attack and occupation.
FDR had to deal with that menacing prospect -- and he did.