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 has named part of another nation’s government an official threat.
The designation imposes wide-ranging economic and travel sanctions on the Islamic Revolutionary Guards Corps as well as organizations, companies and individuals with ties to it. Iran’s Revolutionary Guards carry out operations across the Middle East, train Arab Shiite militias and oversee businesses in Iran.
Monday’s designation “will significantly expand the scope and scale of our maximum pressure on the Iranian regime,” Mr. Trump said in a statement.
The designation, which was opposed by some top Trump administration national security officials who said it could incite retaliation by Tehran against American troops and intelligence officers, takes effect April 15. But it was announced Monday in what American officials described as a chaotic and rushed process.
In Baghdad, where some Iraqi officials have close ties to Iran, American officials said they had been given no guidance on how to enforce the policy.
The timing of Mr. Trump’s announcement appeared aimed at giving Prime Minister Benjamin Netanyahu of Israel a final boost in a tight re-election campaign before a vote on Tuesday. Mr. Netanyahu and Trump administration officials have said Iran is the greatest threat to Israel, and Mr. Nentanyahu immediately thanked Mr. Trump in a tweet.
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.
In thanking Mr. Trump on twitter, Mr. Netanyahu took credit for the idea of the terrorism designation. “Once again you are keeping the world safe from Iran aggression and terrorism,” Mr. Netanyahu wrote in English, while writing in a separate tweet in Hebrew, “Thank you for accepting another important request of mine.”
Last month, in an effort to bolster Mr. Netanyahu, Mr. Trump recognized Israeli sovereignty over the Golan Heights; the United Nations considers it occupied territory.
After Mr. Trump’s announcement, Iran’s Supreme National Security Council said it was designating as a terrorist organization the United States Central Command, which oversees American military operations in the Middle East. Iraqi leaders had no immediate reaction, although Karim al-Nuri, a senior figure in the Badr Organization, an influential Iraqi Shiite militia, said there may not be much direct effect.
“But it will put more pressure on Iraq” from Tehran to grow closer to Iran, he said.
Some American officials said the broad designation potentially covers 11 million members of the Iranian group and affiliated organizations, including the large Basij volunteer militia.
In its statement about the new designation, the State Department also criticized the Quds Force, an elite unit of the Revolutionary Guards led by Qassim Suleimani. That unit and Mr. Suleimani are already under sanctions from lesser terrorism designations.
Top American intelligence and military officials, including Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, opposed Mr. Trump’s action, which they argued would allow Iranian leaders to justify operations against Americans overseas, especially Special Operations units and paramilitary units working under the C.I.A.
But Secretary of State Mike Pompeo and John R. Bolton, the national security adviser, pushed for it, a Trump administration official said. The fighting among the senior administration officials intensified after The New York Times disclosed the pending designation last month.
The final decision belonged to Mr. Pompeo because the State Department controls the list of designated terrorist organizations.
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. The Defense Department says at least 603 American personnel were killed in Iraq by Iran-backed fighters — 17 percent of all Americans killed in hostile action there.
When the Islamic State, a radical Sunni group, took over large parts of Iraq and Syria in 2014, the Quds Force helped train Iraqi Shiite militias to retake the territory. The American military also took part in the campaign, meaning the Americans and Iranians were on the same side.
Many Iraqi officials are opposed to the new terrorist designation, as it could impose travel limits and economic sanctions on some lawmakers in the Shiite-led government and other Iraqis who have ties to Iranian officials.
The new pressure from the Americans also could fuel a popular proposal among Iraqi lawmakers to limit the movements and actions of some 5,000 American troops based in Iraq.
In recent days, State Department officials had asked Mr. Pompeo to delay any announcement, arguing that the designation could have unintended consequences for unrelated countries, a government official said. But Mr. Pompeo dismissed their concerns.
Under a provision of the USA Patriot Act, low-level officials are empowered to deny entry to foreigners who are associated with an organization that the officials decide meet broad standards for terrorism — even when the United States government has not formally applied a terrorism label to that group.
Until now, American officials had never interpreted laws as permitting them to deem a government entity a terrorist organization.
The Trump administration’s decision to breach that constraint with the Iranian group raises the question of whether other government intelligence services that use violence — including those of Israel, Pakistan and Russia — also now meet that standard. If so, should American officials deny visas to people who work with those agencies? State Department officials say the rushed announcement meant such policy details have not been worked out.
The George W. Bush administration considered a range of tough actions on Iran during the Iraq war, but held back.
“The potential blowback vastly outweighs the benefits,” said Jeffrey Prescott, a senior Middle East director under President Barack Obama.
Wendy R. Sherman, a former top State Department official, said the Obama administration considered designating the Revolutionary Guards a foreign terrorist organization, but decided against it because there would be no practical payoff given the risks to Americans and the fact the group was already under other sanctions.
“By designating a foreign military as a foreign terrorist organization, we were putting our troops at risk, particularly our troops in Iraq, next door to Iran,” she said.
The Revolutionary Guards oversaw the previous Iranian nuclear program, and some of the top officers were placed under United States and United Nations sanctions.
The Obama administration entered 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 crackdown against Iran.
There are signs that Trump-era sanctions on Iran are undermining its budget for foreign military activities, meaning less money for Hezbollah and other Iranian allies. The new terrorism designation could expand on this by further undermining the Revolutionary Guards’ finances.