With Iran defying the international voices and sanctions, US, its allies, and UN are all going to be in a difficult spot.
Iran is a powerful nation, in terms of Oil resources, and its influence on the neighboring muslim countries.
Attacking Iran could spark something as disastrous as a multi-country war, which would be seen by muslims as "Islam VS West". This could destabilise the oil industry across the globe. Moreover, Iran is not a weak nation like Iraq, which US could crush in 1 month. It has the 8th biggest defence force in the world, and the IRG whose head count is not known.
From US perspective, it is quite obvious that the next step for Iran would be making nuclear weapons, which they will be able to do in 2 yrs or so !
