Iran has already declared war on the US.
According to the Jerusalem Post, Iran's supreme leader Khamenei has already declared war against the US back in July:
"We are at war with the enemy," Iran's Supreme Guide Ali Khamenehi told a meeting of mullahs in the city of Hamadan, west of Teheran, last Monday. "The central battlefield [of this war] is Iraq."
http://www.freerepublic.com/focus/news/1168644/posts?page=20#20
Iran has already declared its own preemptive strike doctrine, according to ABC News Online:
Iranian Defence Minister Ali Shamkhani has warned that Iran might launch a preemptive strike against US forces in the region to prevent an attack on its nuclear facilities. ABC News reported:
"We will not sit [with arms folded] to wait for what others will do to us," Mr Shamkhani told Al Jazeera television when asked if Iran would respond to an American attack on its nuclear facilities.
"Some military commanders in Iran are convinced that preventive operations which the Americans talk about are not their monopoly.
"America is not the only one present in the region. We are also present, from Khost to Kandahar in Afghanistan; we are present in the Gulf and we can be present in Iraq.
"The US military presence [in Iraq] will not become an element of strength [for Washington] at our expense. The opposite is true, because their forces would turn into a hostage" in Iranian hands in the event of an attack, he said.
http://www.abc.net.au/news/newsitems/200408/s1180031.htm
And the Supreme Leaders security advisor, Hassan Abbassi said:
There are 29 sensitive sites in the U.S. and in the West. We have already spied on these sites and we know how we are going to attack them.'
http://www.memri.org/bin/articles.cgi?Page=archives&Area=sd&ID=SP72304