There's three options that I can see: a ground invasion or strikes on critical infrastructure, or both.

The most that a ground invasion could do is secure a beachhead to be able to strike the interior of Iran more easily. Maybe a temporary airfield or something. Even though the US keeps moving a few thousand troops into the area every few weeks, the idea of taking Tehran with a force that size is pure fantasy. The invasion of Iraq took 300,000 troops in 2003 and was staged across an open desert border in Kuwait to roll across more open desert. On the other hand, Iran is surrounded by mountains on all four sides. You are not going to conquer a country like that with a tenth of the force. On top of that, the military industrial complex hasn't had enough time to come up with good FPV drone countermeasures.

Alternatively, the troops could be staging for raids on Natanz or Fordow or whatever nuclear sites they hyped up recently. There is also the possibility of linking up with belligerent ethnic factions in Iran to balkanize the country.

Option 2 is what Trump threatened - kill the power and water in Iran. There is virtually nothing the Iranians can do about this. The military is obviously the best-equipped entity in Iran for utilities shortages, but the surface world will quickly descend into humanitarian chaos, and their days are numbered at that point. Naturally, the downside is also the humanitarian chaos. Millions of people will be at risk, and we will see yet another Israel/US-provoked wave of refugees.

A very small silver lining in this scenario is that Iranian chicks can actually be pretty hot. They are the ancient Aryans, after all. And that's about it for the upsides.

edit: after considering more perspectives, the Iranians do have one last card to play if we go for their critical infrastructure: destroying the desalinization plants and oil fields of the Gulf oil states. This won't save them, but it will make Trump's win into a classic Pyrrhic victory, to say the least. Oil prices will go absolutely bonkers, and so will the economy.