I think they knew eventually the US would be attacked by either Germany or Japan.
Honestly, I think they were waiting for half of of the commie Ruski population to be killed off in war so that Germany would be properly weakened before entering. Pearl Harbour was just the excuse. Former US presidential candidate seems to agree with me, for what I've read from him.
"The achievements of that organization are monumental, (America First Committee, an anti-war organisation)" writes Buchanan. "By keeping America out of World War II until Hitler attacked Stalin in June 1941, Soviet Russia, not America, bore the brunt of the fighting, bleeding and dying to defeat Nazi Germany."