Given the ideological differences between the Sunni militant group al-Qaeda and the theocratic Shia government in Iran, recent allegations that the two groups had conspired in a foiled attack in Canada struck many observers as odd.

But two years after the death of al-Qaeda leader Osama bin Laden, experts in security and Islamist radicalism say Western intelligence agencies can't afford to discount this possible connection.

Tarek Fatah, founder of the Muslim Canadian Congress and author of the book Chasing a Mirage: The Tragic Illusion of an Islamic State, says that by dwelling on the inherent differences between fundamentalist Sunnis and Shias, Western governments and media have overlooked an obvious similarity.

Radicals on both sides believe "that the world should be under a caliphate," or Islamic state, he says.

"The difference is, will it be under the Islamic doctrines of the Shia or the Sunni? That's a long-term division. But both agree on the doctrine of jihad."

Last Monday, the RCMP arrested Chiheb Esseghaier of Montreal and Raed Jaser of Toronto, accusing them of planning an attack on a Via passenger train and alleging that the plotters had support from al-Qaeda in Iran.

Over the years, the Iranian government has been implicated in attacks carried out by Shia extremist groups, including Hezbollah in Lebanon, but many commentators expressed disbelief that Iran would ever collaborate with a Sunni group such as al-Qaeda.

"I think it would be extraordinarily foolish to ignore the obvious manifestations and likelihood of further co-operation between the Iranian regime and Sunni Islamic extremists, including al-Qaeda," says David Harris, an Ottawa-based lawyer and director of Insignis Strategic Research.

The Sunni-Shia schism

The roots of the Sunni-Shia schism lie in the question of succession after the death of the Prophet Muhammad in the seventh century.

Shias maintain that by not choosing the Prophet's son-in-law, Ali, as successor, Sunnis had defied the Prophet's will, and thus do not represent true Islam.

Because Saudi Arabia houses the Islamic holy sites of Mecca and Medina, it is considered to be the spiritual home of Sunnis, while Iran serves the same function for the Shia. This also largely explains the historic enmity between the two countries.

Examples of Islamic sectarian violence can be seen in places such as Pakistan, Yemen, Bahrain and especially Iraq, where Saddam Hussein’s persecution of Shias during his 24-year reign led to reprisals against Sunnis under Nouri al-Maliki’s Shia government, and produces sectarian attacks on an almost weekly basis.

Noomane Raboudi, an Islamic expert who teaches in the School of Political Studies at the University of Ottawa, says he has trouble believing there is any collusion between Iran and al-Qaeda.

"For the jihadists, the Shia are not even Muslims — [al-Qaeda] almost have the same political view of the Shia as they do of Western people," says Raboudi.

Fatah says that many people in Western institutions such as government, academia and the media have fallen for the "narrative" that because of a history of sectarian bloodshed, radical Sunnis and Shias couldn't possibly collaborate.

He says it's true, for example, that bin Laden viewed the Shias as non-Muslims, but bin Laden also considered the establishment of a global Islamic state more important than dwelling on sectarian differences, which is why he never fought the Iranians.

"Osama might never have collaborated with Iran, but the Iranians were co-operating with al-Qaeda," Fatah says.

Al-Qaeda scattered after 9/11

In an effort to root out the planners of the Sept. 11 attacks, the U.S. declared war on Afghanistan in 2001. As a result, a number of senior al-Qaeda members who had been the guests of the Taliban in Afghanistan took refuge in neighbouring Iran, according to Seth Jones, author of the book Hunting in the Shadows: The Pursuit of al-Qaeda since 9/11.

As further proof of Iranian co-operation with al-Qaeda, Fatah cites an example involving Ahmed Said Khadr, the late father of Canadian-born extremist Omar Khadr and a known al-Qaeda operative. According to Michelle Shepherd’s 2008 book, Guantanamo's Child: The Untold Story of Omar Khadr, Ahmed drove his daughter, Zaynab, to Tehran so she could marry another al-Qaeda member.

In an article for the U.S. news site PJ Media in 2011, former CIA operative Brian Fairchild wrote, "One of the key aspects of Iran's foreign policy has been to undermine the U.S. and its allies in the Middle East by supporting Sunni terrorist organizations such as al-Qaeda, Hamas, and the Palestinian Islamic Jihad."

Despite some philosophical ideological differences, Harris says that Iran and a group such as al-Qaeda have a common cause in their defiance of the West and opposition to the state of Israel.

The fact that Iran may be close to having developed a nuclear bomb would also be of great interest to al-Qaeda, says Harris.

He says if the Shia Islamists that head up the theocratic regime in Tehran "feel they’re in a cosmic battle" against the West, then "it's not surprising that they might link up with the dreaded Sunni in order to fight the first round — and then maybe do in the Sunnis at an appropriate moment."