Best case scenario, the Japanese would need more early success against the US Navy, would need to raise, train, and equip another huge land army, take the western coast (probably just parts of California), fight its way across 1000 miles of altiplano, hundreds more miles to reach the Mississippi...
What always amazed me is the sheer lack of preparation they had before kicking off the shootout with us and the rest of the western powers. They didn't squirrel away inventories of critical resources beforehand. They worked with the principle as they had with their conquests in Asia, just loot and add inventory as they went, which is tenacious as it gets since there is no contingency for error or setback. Simply seize what they needed as they went across the Pacific, but that entailed gathering it, shipping it back to Japan, processing, manufacturing before arriving at a finished product of use. All complicated by another aspect of ill preparation - lift capacity. They had what they had for shipping and were not positioned for wartime manufacturing compared to us. New construction was feeble with long timetables, fraught with QA/QC issues. The loss of a ship, ANY ship, was a real setback, at times having severe impact on particular plans or strategies. Even damage was almost as good as a sinking since their shipyards were terribly slow and less than competent at addressing repairs.
The Japanese attack on the US didn't make sense then, doesn't make sense now, and can't be made to make sense.
But it made sense to
them, from their perspective. They had done their homework, knew the Euro powers were in no position to defend their holdings - the massive loss of life during the First World War and the economic collapse that swept the world afterwards had seen to that. The last people left who could be deemed as an existential threat to their plans for economic & political domination of the Asia/Pacific hemisphere was us. We were another faction of the Caucasian West that were set to be driven out and blocked from having any presence or influence. We were already on their crap list, after not helping them with claims before the League of Nations against Euro powers in the Pacific in the aftermath of WW1, and blocking Japanese immigration in the early 1920s. Those two issues got the Japanese public stirred up, as exploited by opportunist politicians and military, souring relations and getting us crossed off their friends list. Then there was the philosophical component that is largely missed or else deliberately downplayed by some sources. An adherence to religious and societal ideals which drove policies that were way out of step with the ethics of the rest of the world. A following of a sort of manifest destiny, a belief of
hakko ichiu, a philosophy of superiority with a destiny to rule the world. Which is what ultimately led to their fall since they could not competently strike a balance between power and responsibility in a modern-day world. Their values wouldn't allow for it. Consequently they became just another batch of bad guys of history that didn't play well with others who had to be beaten into civility.