This article from the Washington Post gives a good picture of the role of Syria in the transit of jihadis into Iraq. Some interesting insight here:
Worried that it would be Washington's next target, Syria opposed the military coalition invading its neighbor. State media issued impassioned calls for "resistance." The nation's senior Sunni cleric, Grand Mufti Ahmad Kaftaro, undid his reputation for moderation by issuing a fatwa endorsing suicide attacks.
In Aleppo, Abu Ibrahim went door to door encouraging young men to cross the border. Volunteers boarded buses that Syrian border guards waved through wide-open gates, witnesses recalled. Saddam Hussein's government embraced the volunteers, handing them weapons and calling them Arab Saddam Fedayeen. But ordinary Iraqis were often less welcoming, pleading with them to go home; some Syrians were shot or handed over to the invading Americans.
At the request of his Iraqi counterparts, Abu Ibrahim stopped ferrying fighters for a time. "They said there were Shiites everywhere, Americans, and they couldn't do anything." By the summer of 2003, however, the insurgency began to organize itself, and the call went out for volunteers. Safe houses were established. Weapons were positioned. In the vast desert that forms the border with Iraq, passages through the dunes long used to smuggle goods now were employed to funnel fighters.
"We had specific meeting places for Iraqi smugglers," Abu Ibrahim said. "They wouldn't do the trip if we had fewer than 15 fighters. We would drive across the border and then into villages on the Iraqi side. And from there the Iraqi contacts would take the mujaheddin to training camps." Because Syrian men already had served two years of compulsory military service, most of them skipped the training. "It's mostly the Saudis who need the training," Abu Ibrahim said.
Afterward, the fighters were sent to join small cells usually led by Iraqis. They planted booby traps for U.S. convoys and laid ambushes. "Once the Americans bombed a bus crossing to Syria. We made a big fuss and said it was full of merchants," Abu Ibrahim said. "But actually, they were fighters."
In the summer of 2004, Abu Ibrahim got to go to Iraq. He crossed the dunes with 50 other volunteers, dodging U.S. patrols on the Iraqi side. In Iraqi society he moved without drawing attention. He would not discuss much of what transpired during the subsequent months. But when he returned to Syria after the massive U.S. offensive in Fallujah, only three people were alive from the original 50, he said. One was a suicide bomber.
"Young men are fighting with zeal and passion," Abu Ibrahim said. "There are Saudi officers, Syrians, Iraqis. But not those who fought for Saddam. The man who is leading it for the most part is Zarqawi."
As American operations become more frequent in the west of Iraq, Syria is going to either have to clamp down hard on people like Abu Ibrahim or risk having American troops visit on a regular basis the Syrian towns and cities close to the Iraqi border. One way or another the infiltration of men and material into Iraq through Syria is going to have to stop.