The US only joined the war opportunistically and because Japan was stupid enough to drag them into it.

Not to defend Japan, they had it coming, but that part of WW2 was provoked by US as 100% classic interimperialist war for at least West Pacific and maybe even entire East Asia (and in fact Lenin predicted it in 1918, although he predicted it way earlier than it really erupted). At the point US embargoed Japan, it was already fully invested in genocidal brutal war against China and had war materials for half year with zero possibility to disentangle itself.

So they had only two choices: dismantling of their empire which was absolutely unacceptable to them and would probably end up in remnants being swallowed by US or maybe even to face very angry China while having no supplies for armed forces, or take the resources by force which meant conquering Dutch colonies and in this case war with UK and USA was also certain, so their decided to go va banque.

And fuck them, good riddance, but USA had only imperialist motivation for that war and indeed ended up gobbling basically entire Pacific and their own Mare Nostrum in the process, the only thing stopping them even a little was USSR.