The U.S. move was an unprecedented declaration against a foreign government, one that could prompt retaliation and make it harder for Americans to work with allies in the region.
Iran's Supreme National Security Council immediately responded by designating the U.S. Central Command, also known as CENTCOM, and all its forces as terrorist, and labeling the U.S. a "supporter of terrorism."
It was the first time the United States has designated an entity of another government as a terrorist organization, placing a group with vast economic resources that answers only to Iran's supreme leader in the same category as al-Qaida and the Islamic State group.
In Tehran, the semi-official Fars news agency said many of the lawmakers wore the uniform of the guard in a show of support for the force as they convened for a parliament session marking the National Day of the Revolutionary Guard, which follows the lunar calendar. This year it coincides with the April 9 holiday known as Nuclear Day.
Parliament Speaker Ali Larijani denounced the U.S. decision as the "climax of stupidity and ignorance." Supreme National Security Council's spokesman, Keivan Khosravi, said that going forward, "any unusual move by American forces in the region will be perceived as the behavior by a terrorist group." He did not elaborate.
Iranian newspapers carried reports of the U.S. move along with bellicose commentary on their front pages.
The Guard-affiliated Javan daily said any attack on Revolutionary Guard bases and facilities will be "recognized as a right" for Iran to respond. The hard-line Kayhan newspaper said it gave Iranians "permission" to kill American military personnel.
State-owned IRAN daily went a step further, saying the U.S. move was a "designation of the entire Iranian nation" as terrorist.
The pro-reform Shargh daily described it as "the last card" of President Donald Trump against Iran.Trump last year pulled America out of the 2015 nuclear deal between Iran and world powers and re-imposed sanctions on the country, mainly targeting Iran's vital oil sector.
Marking Iran's Nuclear Day, President Hassan Rouhani on Tuesday unveiled dozens of unspecified "achievements" in nuclear technology, something he does every year. This time he touted that beginning of an installation of a chain of advanced centrifuges at the uranium enrichment facility in the central town of Natanz as well as an addition at the Fordo underground facility.
On Monday, U.S. Secretary of State Mike Pompeo said the designation of the Revolutionary Guard is intended to increase pressure on Iran, isolating it further and diverting some of the financial resources it uses to fund terrorism and militant activity in the Middle East and beyond.
But, in addition to the potential for Iranian retaliation, it complicates a delicate balance for U.S. personnel in at least two key countries— Iraq and Lebanon. Iraq has prominent Iranian-affiliated Shiite militias and its government has strong ties to Iran. In Lebanon, the Iran-backed Hezbollah militant group is in parliament and the government.