They might be going in to beef up security at US embassies.The challenge he has is declaring a win and Iran continuing to deny the straights makes him look (more) ridiculous.
Apparently he was briefed on the risk but decided Iran would fold before it became an issue.
Im not sure where Trump wants to drop his Marines. There isn't enough to invade Iran itself, just enough for raiding or occupying a site or two like Kharg island, presumably because he thinks it'll be enough extra leverage to get Iran to surrender for some reason.
Occupying Kharg Island won't get him any more leverage and just leave the troops sitting ducks.
