It's incredible to me how news as this one always provoke debate focusing on military issues. The issues are not those. Who cares about the Iranian air force or the missiles they have? The real issue is political and diplomatic.
If you US attacked Iran you'd have three immediate results:
1. Complete political isolation, with the exception of Israel, alienation of even long-time allies
2. Surge in terrorism against US targets
3. Long term economic effects on energy prices, increase in military and security expenditures
The US acted unilaterally in Iraq and caused one of the biggest political and military debacles of the country's history, severing relations that had been built in 50 years. This coupled with various terror attacks or attempted terror attacks around the world.
Attacking Iran with no reason at all will not only turn every single Muslim in the world against this country (and rightfully so), but alienate every single ally the US has now. No country in the world apart from Israel would ever condone something like this. People didn't like Saddam, and many countries in the area were in fact relieved to see the dictator gone. But this time it would be different: it would be a unilateral attack on a sovereign state.
People in Europe would start asking their governments to close US bases on their soil (they already did in a few) and the US would not only be severely hurt from an operational standpoint, but completely isolated from a diplomatic perspective. 50 years of good diplomatic relations with countries around the would would be wiped out in a month.
When 9/11 happened, the whole world was ready to stand with the US. The operations in Afghanistan didn't even raise an eyebrow around the world. 6 years later the picture is quite different because of Iraq. An attack on Iran would cause complete isolation. And the inevitable series of terror attacks on the US would attract very little sympathy. This time nobody would be allowed to ask "why do they hate us?" on the front pages of newspapers. And no matter how a military is strong, no country survives without allies. A unilateral way of thinking has always caused empires to eventually collapse.
And then there's the economic aftermath, which is not even imaginable.
The only good point is, not even this current administration is stupid enough to do something like this.