Iran has already given a stern warning to America to stay out of Syria and has been strongly allied with Syria since 1979, and remains as strong as diamond to this day.
Since the start of the alliance, they have shaped the politics of the Middle East, and thwarted the regional goals of the U.S. Israel, and Iraq.
If the U.S intervenes and Iran gets involved in the conflict, China and Russia will subsequently get involved in the conflict.
Chinese president Hu Jintao has already stated that if the U.S or any country attacked Iran they would take responsive measures immediately.
As for Russia, they already have already mobilized forces in case of an attack on Iran.
Iran getting involved in any conflict will give an excuse for Israel, which is in possession of nuclear weapons, to attack, which has long been predicted as the precipitating cause to World War 3 due to the subsequent events that are sure to follow as a result.
Iran has also increased its defense budget by 127 percent this year. They are not playing and are ready to fight.