If the US attacked the Iranians, it would make Vietnam seem like a picnic, with the Chinese and Russians backing the Iranians. There is no way they would accept US dominance in their own back yards. Once it happened all bets would be off. Self preservation would guarantee local, regional and world powers in the area would get involved and escalation would happen with unintended and unforeseeable consequences.
The Americans need to think long and hard about where they are going with this, and tbf, I think they have stepped back from retaliating the drone downing because, regardless of their bluster, they realise this.
This now leads to another problem for Trump and he will now, with hindsight, see the reasons why Obama chose to negotiate the treaty with Iran rather than attack them. Having voluntarily withdrawn from the containment treaty, he finds himself with zero leverage or trust with the Iranians. The Iranians have called his bluff by shooting down his drone without retaliation. I predict the Iranians will restart their uranium enrichment programme very shortly and I expect the US to be able to do nothing about it beyond threats and sanctions which are pretty much maxed out anyway.
This has been a humongous blunder by Trump and the US, and the Iranians will take full advantage of it.