The New York Times reports: Not so long ago, Syrians worked to send weapons and fighters into Iraq to help Sunnis fighting a sectarian conflict; suddenly, it is the other way around.

A belated celebration of the Prophet Muhammad’s birthday on the outskirts of this western Iraqi city on Saturday quickly took on the trappings of a rally for Syria’s rebels. Young boys waved the old green, black and white flag Syria adopted in the 1930s after declaring independence from the French. Others collected money to send aid and weapons to the fighters opposing President Bashar al-Assad’s government across the border.

“I wish I could go there with my gun and fight,” said Sheik Hamid al-Hais, a tribal leader interviewed at his compound in Ramadi, the capital of Anbar Province.

It is increasingly clear that Syria’s sectarian war is becoming the regional conflict that analysts have long feared. The rush of recent events — including bombings and assassinations in Damascus and Aleppo, and intensifying violence in northern Lebanon coming directly out of the sectarian hostilities in Syria — suggest that the Assad government now also faces antagonists across its borders.

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.”