Australia was cut off from the USA by the Japanese.
https://en.wikipedia.org/wiki/Proposed_Japanese_invasion_of_Australia_during_Wor ld_War_II
The Japaneses were indeed a formidable force in the Pacific. It was not just Pear Harbor that was attacked that fateful December day. The Philippines were attacked. It was across the date line so technically it was Dec 8th there.
We knew the Japanese were real killers as the killed around 6 million Chinese, Indonesians, Koreans, Filipinos, and Indochinese. We knew they were not squeamish about killing innocents. Any we had to fight them with one hand, while the other hand was tied up in Europe.