WASHINGTON – President Donald Trump defied world powers on Friday and moved to decertify the Iran nuclear deal – an expression of disapproval that America’s strongest allies warned could eventually unravel the landmark agreement and bring the US closer to another conflict in the Middle East.

But another move Trump took on Friday could have a far more sweeping impact on US foreign policy in the Middle East: Labeling the Islamic Revolutionary Guard Corps as terrorists under an obscure Treasury Department designation.

At 1:30 p.m., the Treasury's Office of Foreign Assets Control designated the IRGC under its 13224 terrorist category, a senior State Department official told BuzzFeed News. In a speech ahead of the designation, Trump heralded the move – the first time Washington has labeled the military branch of another country as terrorists.

Given the expansive role the IRGC plays in Iranian society, designating the group as a terrorist organization could have an array of political, military and economic ramifications, experts said.

"This is reckless beyond the extreme," said Barbara Slavin, an Iran expert at the Atlantic Council. "The reason being is that to designate the armed forces of another country as terrorists is to invite retaliation. Would the designation mean that US drone attacks on IRGC personnel are fair game? If so, expect to see Iranian proxies start killing US military personnel again in Iraq or in Afghanistan or Syria."

The heads of the US's closest European allies offered measured criticism of Trump's moves and pledged their continued support for the Iran deal, which was negotiated over years by Iran on one side and the governments of the US, France, Great Britain, Germany, Russia and China on the other.

In a joint statement, Britain, Germany and France said they believed the nuclear deal, known as the Joint Comprehensive Plan of Action, was critical to the world's security and that they would continue to support it. "We encourage the US Administration and Congress to consider the implications to the security of the US and its allies before taking any steps that might undermine the JCPoA, such as re-imposing sanctions on Iran lifted under the agreement," the statement said.

But it acknowledged Trump's concerns about other Iranian actions, noting specifically that Iran's ballistic missile program and other activities in the region also affect European security issues. "We stand ready to take further appropriate measures to address these issues in close cooperation with the US and all relevant partners," the statement said.

Iranian President Hassan Rouhani reacted angrily, promising to retaliate if the US tries to reimpose sanctions. "Iran will not hesitate to give them a fitting response," he said in a speech to the nation. He called Trump's position "incorrect" and pledged that it had made his country more united. He noted that no clause in the agreement allows for its cancellation.

In a briefing to reporters on Thursday, Secretary of State Rex Tillerson said the actions against the IRGC were necessary to curb Iran's aggressive behavior in the region.

“These will be targeted sanctions against individuals, possible entities that are owned or partially owned by the IRGC that are directly supporting these terrorist activities whether its weapons exports, or weapons components, or cyber activities or its movement of weapons and fighters around," he said.

"The Treasury secretary’s going to have broad latitude to being able to identify those and impose additional sanctions," he added.

Once an ideologically hardline entity devoted to the preservation of the values of Iran's 1979 revolution, the Revolutionary Guard has become a major branch of Iran's armed forces, with an estimated 150,000 land, aerial and naval personnel. Though its leaders and officers tend to be ideologically strident and hardline, its rank and file members are draftees from all walks of life.

Since the end of the Iran-Iraq war, the IRGC and its veterans have increasingly become involved in business ventures, playing a significant role in several sectors, including energy, construction, public works and travel. Its construction arm, headquartered at the Khatam ol-Anbia base near Tehran, is among the most powerful economic players in Iran. Its contracts and those of its subsidiaries include the stalled "Peace" pipeline connecting Iranian gas fields to Pakistan and India and the sole subway line in the eastern city of Mashhad, Iran's second largest city. It has built massive highways and dams. In recent months, Khatam ol-Anbia was awarded numerous large projects, including a railway between the northwestern Iranian cities of Tabriz and Orumieh, a titanium production plant and several roadways. It claims to work with 5,000 private companies and subcontractors supposedly employing 135,000 people.

The Revolutionary Guard includes the Quds division, which conducts clandestine operations abroad. But in recent years the Iranian army, too, has deployed military personnel abroad, including controversially to the front lines in support of Bashar Assad's forces in Syria.

Rumors that the Trump administration planned to label the IRGC a terrorist organization resulted in rallying support for the institution from some of the IRGC’s harshest domestic critics. "Parliament will take steps to support and defend the Guards whenever it feels they are under threat," said reformist lawmaker Kamal Dehqan Firouzabadi, according to the Tasnim news agency.

Even the country's relatively moderate president, who has frequently and publicly clashed with IRGC leaders over their meddling in the economy, domestic politics and foreign affairs, backed the armed forces branch.

"They think the IRGC is a military unit. The IRGC is not a military unit; the IRGC is in the hearts of the people," Rouhani was quoted as saying on state television on Wednesday. "The IRGC has always defended our national interests in the face of danger."

The main backers of a blanket terrorist designation of the IRGC include John Bolton, the former US ambassador to the UN, and Mark Dubowitz, the executive director of the Foundation for Defense of Democracies, a neoconservative think tank whose backers have included casino magnate and GOP financier Sheldon Adelson, among others.

Dubowitz, who has long sought the destruction of the Iran nuclear agreement, has become newly relevant in a US administration seeking more ideas for ways to clamp down on Iran. He has met repeatedly with top White House and State Department officials, including Brian Hook, the director of policy planning and Tillerson’s de facto right-hand man.

The move the Trump administration is taking today is technically distinct from labeling the IRGC a Foreign Terrorist Organization under the State Department authority that includes Hezbollah and al Qaeda. In his press conference on Thursday, Tillerson said such a designation brought more risks than the Trump administration was willing to accept.

“There are particular risks and complexities to designating an entire army, so to speak, of a country where that then puts in place certain requirements where we run into one another in the battlefield that then triggers certain actions that we think are not appropriate and not necessarily in the best interest of our military,” Tillerson said.

But Suzanne Maloney, an Iran expert at the Brookings Institution, said that although the Treasury and State Department designations are technically not the same thing, it's a "distinction without a difference." Maloney added that Trump's decision would almost certainly prompt a retaliatory move from Iran. "They always reciprocate," she said. "We’ll have to watch and wait and it’s something that will play out over time."