U.S. intelligence has intercepted messages from Iranian General Qasem Soleimani, head of the Revolutionary Guard's elite Quds Force. The messages, headed towards Iranian-sponsored Shia militias groups in Iraq, informed the groups that they must be prepared to respond with "force" after a U.S. strike on Syria, according to U.S. officials. News of the intercepts has unnerved many U.S. officials, who are now preparing for possible reprisals by Iran and its allies in the region. It's believed that the U.S. embassies in Baghdad and Beirut may be hit by the militia groups, and that Americans in the region may become a target.
The U.S. has imposed sanctions on Iran, and these sanctions have made life difficult for the Iranian people. Both the U.S. and Israel have consistently threatened war with Iran. On top of this, Iran suspects both countries are behind attempts to halt their nuclear programs, which includes the assassination of nuclear scientists. Many in the West, as well as in Iran, believe Iran, rather than Syria, is real target. Talk of removing Assad really is centered on removing the Iranian regime. As the United States is refusing to meet with Iran to discuss Syria, this suspicion seems increasingly more likely.
The Quds Force specializes in asymmetrical warfare. It's likely that U.S. targets would include U.S. embassies. The idea is to deter the U.S. from taking any further action or risk further retaliation, assuming the U.S. goes beyond its initial mandate of "limited strikes" and does indeed intend to tip the balance in favor of the opposition or attempt to remove Assad's regime. It's likely that Iran and the militias would move onto U.S. military bases and possibly target U.S. troops if they engage in conflict.
It doesn't have to be this way. It is not entirely true that Iran is wedded to the Syrian regime. Iran does have contingency plans in place if the regime falls, which include using militia groups inside Syria and enacting contacts with social and civil groups in Syria. Despite talks of clashes between hard-liners and moderate forces within the Iranian government, the groups are driven by self-interest rather than ideology. The clashes between both camps often reflect the different interpretations of the domestic and international climate. The Iranian government urged the Syrian government to talk to the opposition and try to include them in the government and implement the necessary reforms in 2011. Iran used to have a direct channel to the opposition, and there is considerable debate in Tehran over the Syria policy.
Tehran is not beyond rapprochement over Syria. If the U.S. engages in meaningful diplomacy with Iran, they could get Tehran to loosen its ties to Damascus, which could potentially lead to regime change there. However, this can only be achieved through diplomacy, not by bombs or the threat of war with Iran. If this had happened earlier, there would be no threat from Iran now.