WASHINGTON — President Trump said on Monday that he was designating a powerful arm of the Iranian military as a foreign terrorist organization, the first time that the United States had named a part of another nation’s government as such a threat and raising the risk of retaliation against American troops and intelligence officers.
The move, which has been debated at the highest levels within the administration, was imposed on the Islamic Revolutionary Guard Corps. The military unit has carried out operations across the Middle East, trained Arab Shiite militias and taken part in a wide range of businesses in Iran.
The designation “underscores the fact that Iran’s actions are fundamentally different from those of other governments,” Mr. Trump said in a statement. “This action will significantly expand the scope and scale of our maximum pressure on the Iranian regime. It makes crystal clear the risks of conducting business with, or providing support to, the I.R.G.C.”
The action takes effect on April 15 and imposes wide-ranging economic and travel sanctions on the military unit as well as the organizations, companies or individuals that have ties to it — including officials in Iraq, an American ally. Some American officials said the broad terrorist designation potentially covers 11 million members of the Iranian group and affiliated organizations, including the large Basij volunteer militia. In a statement on Monday, the State Department singled out the Quds Force, an elite unit of the Revolutionary Guard that is led by Qassim Suleimani, as an especially nefarious element.
Top Pentagon and C.I.A. officials oppose the designation, which they argue would allow hard-line Iranian officials to justify deadly operations against Americans overseas, especially Special Operations units and paramilitary units working under the C.I.A.
An interagency lawyers group concluded the designation was too broad, but Secretary of State Mike Pompeo and John R. Bolton, the national security adviser, pushed for it, said a Trump administration official. The fighting among the senior administration officials intensified after The New York Times disclosed the pending designation last month.
After Mr. Trump’s announcement, Iran’s Supreme National Security Council said it was designating the United States Central Command, the part of the military that oversees operations in the Middle East, as a terrorist organization.
At the height of the Iraq War in the mid-2000s, Iranian military officials and partners helped train Iraqi Shiite militias to fight American troops. When the Islamic State, a radical Sunni group, took over large parts of Iraq and Syria in 2014, the Quds Force and other Iranian groups helped train Iraqi Shiite militias to work with the Iraqi Army in retaking the territory. The American military also took part in the campaign, meaning the Americans and Iranians were on the same side of the fight against the Islamic State.
Senior Iraqi officials are opposed to the new designation, as it could impose travel limits and economic sanctions on some lawmakers in the Shiite-led government who have ties to Iranian officials. The additional pressure on Iranian groups also could fuel a popular proposal among Iraqi parliamentarians to limit the movements and actions of 5,000 American troops based in Iraq.
Generally, Iraqi leaders say they oppose any sanctions because ordinary Iraqis suffered under broad United Nations economic penalties that were imposed after Saddam Hussein invaded Kuwait in 1990.
Monday’s announcement came one day before the Israeli general elections, and the move on the Iranian group could give Prime Minister Benjamin Netanyahu a boost in the final hours of his re-election campaign. Mr. Netanyahu has repeatedly raised the specter of the Iranian threat to Israel and tried to reinforce the notion that his close ties to Mr. Trump strengthen Israeli security.
After Mr. Trump’s morning announcement, Mr. Netanyahu thanked him on Twitter. “Once again you are keeping the world safe from Iran aggression and terrorism,” Mr. Netanyahu wrote.
Last month, in an explicit effort to bolster Mr. Netanyahu, Mr. Trump recognized Israeli sovereignty over the Golan Heights, which Israel seized from Syria in the 1967 war and annexed in 1981; the United Nations considers it occupied territory.
Mr. Netanyahu has stressed the dangers posed by Hezbollah, which was designated a foreign terrorist organization by the United States in 1997 and has close ties to the Revolutionary Guard. He had asserted recently that Hezbollah was trying to set up a base in the Golan Heights. Last month, after visiting with Mr. Netanyahu in Jerusalem, Mr. Pompeo flew to Lebanon and berated officials for tolerating Hezbollah, even though it is a part of the government in Beirut.
“There is a reason that successive administrations have held off designating the I.R.G.C. as a terrorist organization, and why many of Trump’s own military and intelligence officials are said to be highly opposed to the move: The potential blowback vastly outweighs the benefits,” said Jeffrey Prescott, who worked as a senior Middle East director at the White House National Security Council during the Obama administration.
“This isn’t about taking a tough approach to Iran’s support for terrorism,” Mr. Prescott said Monday. “Rather, it will put our service members in Iraq and throughout the region at additional risk with nothing to show in return.”
The Obama administration considered a series of actions against the Revolutionary Guard before entering into a nuclear deal with Tehran and world powers in 2015. Mr. Trump withdrew the United States from that agreement last year in the start of a series of crackdowns against Iran.
The Revolutionary Guard oversaw the previous Iranian nuclear program, and some of its top officers were sanctioned by the United States and the United Nations for their efforts.
The New Yorker reported in 2017 that the Trump Organization had been involved recently in a hotel project in Azerbaijan whose wealthy backers have ties to Iranians linked to the Revolutionary Guard.