The Quds Force, the elite Iranian unit implicated Tuesday by the Justice Department in an alleged plot to assassinate the Saudi ambassador to Washington, was formed during the 1980s as part of the Revolutionary Guard Corps. Since then, it has evolved into a premier special forces unit responsible for foreign operations, including, U.S. officials have said, attacks on American troops in Iraq.

Named for the Arabic word for Jerusalem, the Quds Force has long been associated with training and equipping Islamic revolutionary groups around the Middle East, notably the Palestinian militant group Hamas in the Gaza Strip and the armed Shiite group Hezbollah in Lebanon.

During Iran’s 1980-88 war with neighboring Iraq, the Quds Force helped Iraqi Kurds fight Saddam Hussein as part of the war effort of the Revolutionary Guard, the military branch created to protect Iran’s 1979 Islamic revolution. Like the Revolutionary Guard, the Quds Force is believed to report directly to Iran’s supreme leader, bypassing the president. The current supreme leader, the Shiite Muslim cleric who serves as the country’s top religious and political authority, is Ayatollah Ali Khamenei.

In 1994, a Quds Force commander was suspected of helping to plan and finance an attack by Hezbollah operatives on a Jewish center in Buenos Aires in which 85 people were killed.

But it was after U.S. forces invaded Iraq in 2003 that the force emerged as a shadowy but deadly threat to American troops. According to U.S. generals, the Quds Force supported Iraqi Shiite militias and helped plan a January 2007 raid on U.S. troops in the city of Karbala. Four American soldiers were captured in the raid and later killed.

The Pentagon also charged that Quds Force members were involved in smuggling sophisticated bombs and other weapons into Iraq for use against American troops. In late 2006, U.S. forces in northern Iraq seized two Iranians later identified as top Quds Force officers — Brig Gen. Mohsen Chirazi and Col. Abu Amad Davari.

In February 2007, President George W. Bush publicly charged that “the Quds Force, a part of the Iranian government, has provided these sophisticated IEDs [improvised explosive devices] that have harmed our troops” in Iraq.

But Bush said he did not know whether the force’s orders came from “the top echelons” of the government of President Mahmoud Ahmadinejad. “Whether Ahmadinejad ordered the Quds Force to do this, I don’t think we know,” Bush said. “But we do know that they’re there” in Iraq and that the Quds Force is “a vital part of the Iranian government.”

In talking about assassination accusations revealed Tuesday, U.S. officials said it was unclear who in Iran ordered or knew about the alleged plot.

Attorney General Eric H. Holder Jr. called the Quds Force “a component of the Iranian government” and charged that the plot was “directed and approved by elements of the Iranian government.” But in response to a question, he demurred on whether the operation was known or “blessed” at the very top of the Iranian hierarchy.

“We are not making that charge at this point,” Holder said.

In April, the Obama administration accused the Quds Force of helping the government of Syrian President Bashar al-Assad carry out a brutal crackdown on protesters inspired by the Arab Spring movements. The Quds Force was included on a list of people and entities subject to new U.S. sanctions imposed because of the crackdown.