President Donald Trump is widely expected to label Iran’s elite Islamic Revolutionary Guard Corps (IRGC) a terrorist organization, a move that Tehran and its top fighters said would be met with serious consequences.

Trump has long criticized Iran and its expanding sphere of influence in the Middle East, where both the U.S. and Iran are vying to defeat the Islamic State militant group (ISIS) and affect regional politics. As the Republican leader stirs fury in Tehran over strong indications he will decertify a historic nuclear treaty signed by the U.S., Iran and other leading powers, a spokesman for the Iranian Foreign Ministry warned that Trump’s additional threat to outlaw Iran’s leading force would be a grave error.

“I hope that the U.S. ruling body would not make the strategic mistake…, but if it does so, then Iran’s response will be firm, decisive, and crushing and the U.S. should accept its consequences,” the spokesman, Bahram Qassemi, told reporters, according to the semi-official Tasnim News Agency.

Known members and affiliates of the IRGC are already flagged as terrorists by the State Department, but media outlets including The Financial Times have cited unidentified U.S. officials who suggested the Trump administration sought to take on the group as a whole. After Trump again condemned Iran’s “bad behavior” during an interview with former Arkansas Governor Mike Huckabee that aired Saturday on Christian media outlet TBN, the head of the IRGC himself weighed in on the issue.

“If the scattered news about the stupidity of the U.S. government regarding the IRGC as a terrorist group is correct, the Guards will also consider the American military all over the world, especially the Middle East, as equal to Daesh,” Major General Mohammad Ali Jafari warned Sunday, using the Arabic-language acronym for ISIS. His comments were carried by Press TV, the English-language affiliate of the semi-official Islamic Republic of Iran Broadcasting outlet.

Jafari, who was appointed the commander of the IRGC in 2007 by Ayatollah Ali Khamenei, Iran’s supreme leader, also warned that Iran would respond to additional U.S. sanctions by expanding its missile capabilities, which already pose a threat to most U.S. bases in the Middle East and Afghanistan. In Iraq and Syria, both U.S.- and Iran-backed militias are actively fighting ISIS, and tensions have risen between the two factions on the ground as their leaders exchange threats at home.

The IRGC was developed in the aftermath of the 1979 Islamic Revolution that overthrew U.S.-backed Shah Mohammad Reva Pahlavi and installed Tehran’s current Shiite Muslim government. Hundreds of U.S. diplomats and staff were seized by Iranian revolutionary forces following the uprising and, though the 444-day hostage crisis ended peacefully, the two countries have not maintained regular diplomatic relations since.

While the IRGC is primarily responsible for national security in Iran, its fighters have also been deployed to battle ISIS in Iraq and Syria, and its prestigious Quds Force, led by Major General Qasem Soleimani, has reportedly been involved in numerous operations abroad. In 2007, the U.S. designated the Quds Force a supporter of terrorism, further alienating the two governments.

In 2015, President Barack Obama quietly reached out and ultimately signed a deal with Iran, China, France, Germany, Russia and the U.K. that lifted billions of dollars’s worth of sanctions against Iran in exchange for Tehran significantly cutting its nuclear production. The U.N.’s International Atomic Energy Agency and even the State Department have repeatedly found Iran to be in compliance with the deal, but critics say it did not go far enough.

Source » newsweek