Like Iraq and Afghanistan before it, analysts say, Syria is likely to become the training ground for a new era of international conflict, and jihadists are already signing up. This weekend, Al Qaeda’s ideological leadership and, more troublingly, the more mainstream Jordanian branch of the Muslim Brotherhood, called for jihadists around the world to fight Mr. Assad’s government.

Nowhere is the cross-border nature of sectarian hostilities more clear than in Iraq’s western desert, where Sunni Arabs are beginning to rally to the cause of the Syrian opposition and, in the process, perhaps strengthen their hand in dealings with an antagonistic Shiite-led national government in Baghdad.

A weapons dealer who operates in Anbar, who said he goes by the alias Ahmed al-Masri, said, “Five months ago I was told that the Syrian brothers are in need of weapons. I started to buy the weapons from the same guys that I previously sold to — the fighters of Anbar and Mosul. I used to bring them from Syria; now it’s the other way around.”

The man said he was selling mortars, grenades and rifles, and that his contact in Syria was also an Iraqi. In some instances, he said Iraqis were giving away weapons, and in those cases he charged money only to transport them across the border.

“It’s a good business, but it’s not easy money,” he said. “It’s risky, but this is life.”

Tribal leaders and security officials describe a small but increasing flow of weapons to Syria from Anbar Province and areas around Mosul, the northern city that is a headquarters for Al Qaeda in Iraq. For some weapons smugglers the price of an automatic rifle has increased dramatically — to $2,000 from about $300, according to one account.