You could justify taking out the nukes ability as it is something they are not supposed to have. It would be much more difficult justifying a full scale attack on Iran. If the nukes were taken out and Iran decided to retaliate then they would be the aggressor. A tactical strike is not the same thing as starting a war.
U.S. and Russia discuss flight safety in Syria on conference call (Reuters)
from the article: Report: U.S. positioning 'bunker-busters' for possible Iran strike