The US and Iran may be following parallel tracks in their fight against jihadists of the so-called Islamic State, but decades of mutual hostility – and opposing aims in Syria – are shaping Iranian doubts and conspiracy theories about US motives.

Iran’s top leadership blames the US, CIA, Israel, and America's Sunni allies such as Saudi Arabia for “creating” the IS as a tool to undermine Iran. It's a way to ensure a permanent US troop presence in the Middle East and to create chronic regional tensions that benefit Israel, according to Iran.

Iran’s view of the US - IS relationship is very much framed by the time-honored “enemy of my enemy is my friend” mentality – but in reverse. If both the US and IS are Tehran’s adversaries, then clearly the US and IS must also be friends with each other.

US jets, which began attacking IS positions in Iraq in August, now regularly strike IS positions in Iraq and Syria. And in what has become a highly symbolic battle along Syria’s border with Turkey, the US has carried out more than 135 airstrikes to check the IS advance on Kobane. On Sunday, for the first time, US planes airdropped 27 bundles of weapons and ammunition to prevent Kobane’s embattled Kurds from being overrun.

Still, in Tehran, such efforts are not seen as part of a serious anti-IS fight in which the US and Iran might have similar aims.

“The United States’ military strikes in the region and ISIS terrorist actions carry no truth and are doomed to failure,” Iran’s Qods Force commander, Maj.-Gen. Qassem Soleimani, said Monday of the latest reported strikes, Iranian media reported. The approaches of both the US and IS “are not effective and they will not succeed.”

For its part, Iran has provided advisers and crucial military hardware and support to the fragile Iraqi Army and Shiite militias who are battling IS. Iraq’s new prime minister, Haidar al-Abadi, arrived in Tehran late Monday, and in recent days Soleimani has been photographed along the frontline in Iraq meeting with Kurdish fighters.

Tangled complexities

Indeed, both the US and Iran see a common threat from the Sunni extremists of IS, who boast of their mass killings, hate “infidel” Iran and Shiites as much as they do US-backed Sunni states, and dream of expanding their violent writ from the deserts of North Africa to the steppes of Central Asia.

But the official line here provides insight into the tangled complexities of keeping a long-standing adversary in the category of enemy, despite sharing the same battlefield goal.

“If the US can overthrow a government overnight, why can’t it destroy a group that is only a handful of people? It should not be too difficult,” says Mehdi, a bespectacled government worker, as he left Friday prayers in Tehran, a weekly event that attracts several thousand conservative believers.

“They want to destroy the image of Islam, to portray it as a terrorist religion, while the real root of terrorism is the US backed by Israel,” asserts Mehdi, echoing the message of Friday prayer leaders across the country for weeks.

The Syria question

For reasons that sometimes bear little scrutiny, many here point to the expansion of IS in Syria alongside less radical rebels long supported by the US, the West, and their Sunni Arab allies in the Persian Gulf – all of them fighting Syrian President Bashar al-Assad, Iran’s staunch ally – as proof of American duplicity and clandestine support for IS.

The key argument: Washington wants Mr. Assad to go, but Iran says only an intact Syrian state can fight “terrorism,” and that right now there are few alternatives to Assad. If the US were truly determined to vanquish the IS, so the thinking goes, it would drop its opposition to Assad. (Paradoxically, US NATO ally Turkey cites what it says is the lack of a US commitment to depose Assad as a reason it has not joined the fight against IS in Syria more forcefully.)

Even Iran’s centrist President Hassan Rouhani – whose team is engaged in nuclear talks with the US and five other world powers – has blamed “strategic blunders” by the US and the West for lighting the “fire of extremism and radicalism.”

The IS, which in Iran is called by its Arabic acronym Daesh, has a “temporary mission” separate from any plans to expand its territory, suggests Hossein Shariatmadari, editor of the fundamentalist Kayhan newspaper and an official representative of Iran’s supreme leader, Ayatollah Ali Khomeini.

Usually those who want to rule try to present a “good image of itself to people … but Daesh is doing exactly the opposite,” by videotaping atrocities and broadcasting them, says Mr. Shariatmadari. “It’s main goal is to scare people about Islam, so the enemy can make use of these fear-mongering tactics.”

Like many here, he links cash and arms that have been provided to the less extreme anti-Assad rebels with deliberate championing of IS. The US and its Western allies deny having ever provided direct support to IS. They say Iran carries much of the blame for backing the Assad regime, which is responsible for the vast majority of the those killed in Syria – a UN estimate last week put the number of deaths at more than 200,000 – thereby feeding support for IS.

“Daesh is part of a proxy war, so who can deny Daesh is the creation of the West?” asks Shariatmadari. “The stance it takes is exactly in line with the US, and is against the resistance forces of [Lebanese] Hezbollah and Iran. It doesn’t do anything against Israel whatsoever.”

Evidence of the sectarian brew created by the Syria and Iraqi wars, which only sharpens the view in Shiite Iran that the US is allied with Sunni IS, was heard when one Tehran Friday prayer leader, Ahmad Khatami, reacted to the death sentence given in Saudi Arabia for a prominent Shiite cleric, Sheikh Nimr al-Nimr.

“We understand Saudi Arabia’s anger because they have spent billions of dollars to topple the Syrian regime, but they have failed,” said Mr. Khatami, sparking chants of “Death to America!” and “Death to Israel!”

“Who does not know that Daesh was created by [Saudi Arabia’s] petrodollars and every act of terror in the Muslim world is supported by their petrodollars?” said Khatami. “You are not just part of the problem, but the whole problem.”

Inadvertent help?

Iranian officials have called the US-led coalition against IS an “obvious and clumsy deception,” and US airstrikes “irrelevant.” But US and Iranian military efforts, even if inadvertently, help the other.

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Iran’s longstanding support of Shiite militias in Iraq means it has “managed to cultivate intelligence on, and a deep understanding of, IS warfare,” Ellie Geranmayeh wrote in an analysis for the European Council on Foreign Relations earlier this month.

“It is politically difficult for either Washington or Tehran to openly endorse the actions of the other or to actively cooperate together against IS,” wrote Ms. Geranmayeh. Still, US and Iranian efforts in Iraq “have been accepted by both sides as a necessary evil to weaken IS, although the same cannot be said for Syria.”