A Pandora’s box was opened in the Middle East in late May. That was when Yusuf al-Qaradawi, the Egyptian theologian who is perhaps the world’s most influential Sunni cleric, called on Sunni Muslims worldwide to fight against the regime of President Bashar al-Assad and Hezbollah in Syria. In the weeks and months ahead, Qaradawi’s statement will surely quicken the stream of foreign fighters into Syria. Before long, Syria’s civil war could turn into an all-out sectarian conflict involving the entire region.

The 86-year-old Qaradawi is a religious cleric who left Egypt for Qatar in 1961 and has since become something of a celebrity among Islamic religious leaders. He has authored more than 100 books that are sold across the Muslim world, and his weekly TV show on al Jazeera has tens of millions of viewers. Qaradawi owes much of his influence to his careful balancing of populism and political conservatism. He manages to combine, for example, hard-line views on Israel with vigorous condemnation of al Qaeda. He has built a reputation as someone who speaks truth to power, all the while retaining the privileges -- such as a TV program and a professorship -- that come with being close to the establishment. In some sense, he is the closest thing that the Sunni Muslim world has to a pope.

Qaradawi’s controversial remarks fell at a Friday rally in Doha on May 31. In an emotional address about the plight of Sunnis in Syria, Qaradawi declared that “anyone who has the ability, who is trained to fight . . . has to go; I call on Muslims to go and support their brothers in Syria.” That’s a remarkable message, precisely because it is one that clerics of Qaradawi’s stature almost never make. Establishment Islamic clerics often declare that a given armed struggle is a legitimate jihad, but they rarely say that Muslims worldwide have a duty to join it. Radical clerics have been known to make this so-called individual duty argument, which consists of saying that all able Muslim men must fight and that declining to do so would be a sin. But mainstream clerics such as Qaradawi usually make the “collective duty” argument, which implies that outsiders can fight under certain conditions but with no obligation. Even at the height of the very popular Afghan jihad in the 1980s, the Saudi Sheikh Abd al-Aziz bin Baz said only that Muslims have “a duty to support” -- not “an individual duty to fight with” -- the Afghan mujahideen. He left it to more radical figures such as Abdullah Azzam, the ostensible mentor of Osama bin Laden, to argue that all Muslims had to fight.

To be sure, Qaradawi did not use the term “individual duty” (fard ‘ayn in Arabic), and he qualified his call by suggesting that primarily men with military training should go. However, in a region where conscription is the norm, that means practically everyone. By calling on all capable Sunnis to fight in Syria, Qaradawi is not only echoing jihadi ideologues, he is also contradicting his earlier self. In 2009, he wrote a book titled Jurisprudence of Jihad, in which he dismissed the individual duty argument for the jihad in Palestine, Iraq, and Afghanistan. His willingness to make an exception for Syria today is an indication of how strongly Sunni Arabs outside Syria feel about the conflict.

Other clerics have also made the individual duty call for Syria in the past year, but none of them is nearly as influential as Qaradawi. His statement, therefore, has an important norm-setting effect for other clerics: It makes it easier for them to talk tough on Syria and more difficult for them to act like doves. Accordingly, the month of June saw a string of statements by senior clerics across the region calling for jihad in Syria. For example, just days after Qaradawi’s statement, the grand mufti of Saudi Arabia, Abdul Aziz al-Sheikh, publicly endorsed the part of Qaradawi’s lecture that denounced Hezbollah as the “party of Satan.” The mufti did not explicitly address the issue of foreign fighting but made clear that he approved of Qaradawi’s rhetorical escalation. Similarly, a week later, a group of Yemeni ulama (Islamic clergy) released a collective fatwa calling for the “defense of the oppressed” in Syria. Like the Saudi mufti, the Yemeni clerics did not repeat Qaradawi’s call for even non-Syrians to fight, but they did not criticize it, either. Two weeks after Qaradawi’s talk, the Saudi cleric Saud al-Shuraim declared from the pulpit of the Grand Mosque in Mecca that believers had a duty to support Syrian rebels “by all means.” The following day, (recently deposed) Egyptian President Mohamed Morsi spoke at a rally in Cairo, waving the flag of the Syrian opposition and denouncing both the Assad regime and Hezbollah. The rally was organized by hard-line clerics advocating a tougher stance on Syria, and although Morsi did not explicitly endorse foreign fighting, his appearance at the rally was widely interpreted as a nod to those wishing to engage in it.

Taken together, these statements will also produce more Sunni war volunteers for the battle in Syria -- which would be less of a concern if Syria were not already teeming with foreign fighters. According to data that we have collected over the past nine months from hundreds of primary and secondary sources, about 5,000 Sunni fighters from more than 60 different countries have joined the Syrian rebels since the uprising began in 2011. This makes Syria the second-largest foreign-fighter destination in the history of modern Islamism. (In the 1980s, the Afghan jihad drew approximately 10,000 volunteers but over a period of ten years.)

And that is before taking into account the foreigners fighting on the other side. In addition to advisers from the Iranian Revolutionary Guards and Lebanese Hezbollah members, an increasing number of Shiite foreign fighters from Iraq and Lebanon are joining in to help the Assad regime. According to unconfirmed reports, Shiites from Yemen, Afghanistan, and India have started to arrive as well. Many of them say they come only to protect Shiite shrines, such as the Sayyida Zaynab mosque in Damascus, but some of them take part in more nefarious activities. Lebanese Hezbollah were key in retaking Qusayr last month and are now in the battle for Homs. The past month has also seen an influx of Iraqi Shiites affiliated with the two groups Kata’ib Hezbollah and Asa’ib Ahl al-Haq, whose histories go back to the U.S. occupation of Iraq. Both groups have fought Sunni rebels in and around Damascus.

Now, with Qaradawi’s intervention, the arrival of Sunni rebels will likely speed up. It is not that all of his followers will suddenly pack their bags, but a certain number of fence-sitters can be expected to be swayed by his pronouncement. In that way, there is a good chance that there will be more than 10,000 foreign fighters on either side of the Syrian war within a year or two.

The involvement of nonstate military actors on this scale will create a host of security challenges in Syria and beyond. Within Syria, the presence of radical foreign fighters will make peace negotiations and post-conflict reconstruction efforts vastly more complicated. Regionally, there are already sectarian hostilities spilling over from Syria to neighboring Lebanon and Iraq. Further afield, the fledgling democracies in Tunisia, Libya, and Egypt will have their hands full when hundreds of battle-hardened jihadists return home. Western governments in both Europe and North America also have reason to be concerned about Muslim citizens returning radicalized from Syria. The most worrying prospect of all, however, is that states in the region will get more directly involved in the war to defend what they see as their suffering brethren in Syria. This could lead to an all-out proxy war with unforeseeable consequences.

Already, there has been some competition among Sunni states for influence in the Syrian war. Indeed, one way to interpret Qaradawi’s statement is as a symptom of the struggle between Qatar and Saudi Arabia over the patronage of the Syrian rebels. Qaradawi, who is Egyptian but a resident of Qatar, may have been seeking to outbid his more conservative colleagues in Saudi Arabia, most of whom have argued that non-Syrians should not join the fight but rather support it through other means. By talking tough, Qaradawi may have hoped to win influence for his Qatari hosts among Syrian rebels and more followers for himself in the wider Sunni community. We have seen such bidding games in several past conflicts in the Muslim world. For example, in the 1980s, Arab Gulf countries vied for the role as the most generous donor to the Afghan mujahideen, and in the 1990s Saudi Arabia competed with Iran over which country could send more weapons to the Muslims in Bosnia. Needless to say, this is a process that privileges military escalation over diplomatic resolution.

Of course, one should not overstate Qaradawi’s role in the Syria crisis -- there was a fire before he poured petrol on it. However, the May 31 statement serves, at the very least, as an important indicator of Sunni clerical opinion on the Syrian war. It is a sign that what Sunni religious and political elites used to consider rabble-rousing is now entering the mainstream. Although several media commentators, including Sunni Arab ones, have criticized Qaradawi and accused him of encouraging sectarian bloodshed, the condemnation has hardly been overwhelming. Qaradawi himself has shown no intention of walking his statement back. But even if he did, the genie of militant sectarianism is out of the bottle.