Militant Sunnis from Iraq have been going to Syria to fight against President Bashar al-Assad for months. Now Iraqi Shiites are joining the battle in increasing numbers, but on the government's side, transplanting Iraq's explosive sectarian conflict to a civil war that is increasingly fuelled by religious rivalry.
Some Iraqi Shiites are travelling to Tehran first, where the Iranian government, Syria's chief regional ally, is flying them to Damascus, Syria's capital. Others take tour buses from the Shiite holy city of Najaf, Iraq, on the pretext of making a pilgrimage to a Shiite shrine in Damascus that for months has been protected by armed Iraqis. While the buses do carry pilgrims, Iraqi Shiite leaders say, they are also ferrying weapons, supplies and fighters to aid the Syrian government.
"Dozens of Iraqis are joining us, and our brigade is growing day by day," Ahmad al-Hassani, a 25-year-old Iraqi fighter, said in Damascus. He said that he arrived there two months ago, taking a flight from Tehran. The Iraqi Shiites are joining forces with Shiite fighters from Lebanon and Iran, driving Syria ever closer to becoming a regional sectarian battlefield.
Lebanon, which has 100,000 Syrian refugees, was pushed to the brink this month when a Sunni intelligence chief was assassinated in a bombing. Many Lebanese blamed the Syrian government and its allies for the attack. Jordan, sheltering more than 180,000 refugees. has struggled to contain the violence on its border. Turkey, with more than 100,000 refugees, has traded artillery fire with Syria since Syrian shelling killed five civilians near the border early this month.
Now Iraq, still haunted by its own sectarian carnage, has become increasingly entangled in the Syrian war. And Iran, which, like Iraq, is majority-Shiite, appears to be playing a critical role in mobilising Iraqis.
According to interviews with Shiite leaders in Baghdad, the Iraqi volunteers are receiving weapons and supplies from the Syrian and Iranian governments, and Iran has organised travel for Iraqis willing to fight in Syria on the government's side.
Iran has also pressed the Iraqis to organise committees to recruit young fighters. Such committees have recently been formed in Iraq's Shiite heartland in the south and in Diyala province, a mixed province north of Baghdad.
Many Iraqi Shiites see the Syrian war - which pits the Sunni majority against a government dominated by Alawites, an offshoot of Shiite Islam - as a battle for the future of Shiite faith.
This sectarian cast has been heightened by the influx of Sunni extremists aligned with al-Qaeda, who have joined the fight against the Syrian government much as they did in the last decade against the Shiite-led Iraqi government.
"Syria is now open to all fighters, and al-Qaeda is playing on the chords of sectarianism, which will spur reactions from the Shiites, as happened in Iraq," said Ihsan al-Shammari, an analyst and professor at Baghdad University's College of Political Science.