Iran does not have a decent army, navy or air force.+1 for the "iranians could make life hell for the americans" ive been saying this for a while, iran has got a decent army and they will give the americans one hell of a fight it wont be no where near as easy as iraq, as for the hoping britain wont get involved part sadly that aint gonna happen, if america goes to war britain go to war end of.
They have a better than average missile defence system but that can be taken out with little or no loss.
I doubt they are going to invade Iran there would be no point to it.
What may happen is that there will be attacks to gain Air Superiority followed by tactical airstrikes on Air force and Navy. Followed by destruction of Nuclear facilities by long range surface to surface, cruise missiles, drones and B2 stealth bombers.
Even if the unthinkable silly thing happens i.e. boots on the ground, you might as well go crazy and drag Israel and the willing parts of NATO into it. Military wise Isreal have the ability to neutralise any Irananian Assets that they deem a threat, alone (Not without losses). America is the only thing holding Israel back from a strike on Iran. In fact they are not stopping them just pushing the timetable for action back.