Here is a future scenario for you.
World opinion turns against Iran. Western nations form a real coalition. Iraq is conveniently there as a base of attack. Coalition forces use Iraq as a base of attack.
US does most of the bombing.
Coalitions forces stay in Iraq and Iran to keep the peace. More US troops can go home. Republican president wins in 08. Democrats lose again.
Everyone just delays and delays, China and Russia do not get involved, and Iran tests a nuclear device. A blockcade is set. Iran launches a few nukes. Everyone invades Iran for varying reasons. China and US stand off in Tehren. A new berlin wall is created, but mostly in name, as economic trade is healthy.
Coalition forces invade Iran. Oops, no nukes found! World embarassed. Entire middle east simmers with tension. Someone’s capital city becomes nuked.
Iran tests nukes after delay. nobody does anything. because it has nukes, Iran doesn’t do anything and acts more like a first rate country. the country goes democratic anyway and becomes important economically and nobody cares.
Iran tests nukes, remains fundamentally islamic, but doesn’t do much asides from that. It becomes much like Iraq, a thorn on everyone’s side that nobody cares much about. It implodes in civil war and becomes a 3rd world nation barely reaching headines in the news.