1. Of course. The US could beat any other army in the world virtually effortlessly - with the possible exception of Russia and China - because they are such huge countries with respectable militaries and lots of manpower which could prove crucial in any sort of protracted war.
An Iranian/Saudi alliance (which is far from the most likely thing to happen) would put an end to any US invasion plans though. Any war waged against Iran and Saudi Arabia would be horrendously expensive, especially with the military kit the Saudis use, which is VERY up to date. Better keep those two apart.
2. Depends on the circumstances. Britain will certainly back the US on an Iranian invasion, but may not participate on the sort of scale that has been seen in recent years. I am sure Britain would play some sort of role, however minor, in the invasion.
3. Iran have a fairly outdated military. Their armour is poor, old Soviet junk that has shown how well it stands up to the might of the Challenger 2 and M1A2 in Iraq. Their air defence is far more advanced than Iraqs was, certainly bought with the US and Israeli air forces in mind - this will probably be the greatest military deterent Iran possesses, making air strikes more difficult. Their air power is better than Iraqs was, but Iranian pilots have never seemed capable of making the most of their jets.
Bell wrote:
3.They will almost certainly nuke someone, and there, is the problem, what do we do if we turn our televisions on one morning to hear new york/london or any other major city has been wiped out?
How would they manage that?
Even if they completed their nuclear program overnight a delivery system capable of targeting Europe or the US is something they certainly don't have. I think the greatest threat posed by Iranian nuclear proliferation is that they may supply terrorists with nuclear weapons. Imagine if Hamas detonated a nuke in Tel Aviv - that is quite possible, but not yet.