As has been feared for months, violence from the Syrian civil war has spilled across the border into Iraq, threatening an already unstable balance of power in the neighboring country. A group of Syrian soliders were ambushed and killed inside Iraqi territory on Monday, raising concerns that the violent conflicts in both countries could somehow merge.

The Syrian soliders and government employees crossed over the Iraqi border last week to escape a rebel onslaught inside Syria. They turned themselves over to the Iraqi authorities, who refused to let them stay and were escorting the Syrians back across the border when they were attacked. Gunmen ambushed the convoy before they could reach the al-Waleed border crossing, killing most of the Syrians and some of the Iraqi police officers assigned to protect them. The AP says that 42 Syrians were killed, along with seven Iraqis, and that the "well-coordinated" involved guns, rocket-propelled grenades, and machine guns.

It's not clear who the gunmen were affiliated with, but it's possible that they were Iraqi Sunnis sympathetic to the rebel cause in Syria—or at the very least, are anti-Bashar al-Assad. (It's also unlikely that Syrian rebels managed to get across the border themselves.) Sunnis in Iraq's Anbar province have been protesting against their own Shi'ite-dominated government for months, and al-Qaeda in Iraq is still active in the area.

The Sunni-Shia divide stretches from Baghdad to Tehran and back to Damascus, where the government is dominated by Assad's Alawite sect, but is being propped up the Shi'ite leaders of Iran. That, plus the increased fighting along the Syria border, made the Syrian soliders an easy target for retribution.

Syria's longest border is on the western edge of Iraq, where thousands of refugees have fled to escape their two-year-long civil war. The Iraqis had been enforcing a strict policy of not allowing any men to cross, even those who were with their families. Iraqi officials had allowed the Syrian group to escape their battle with the rebels, but refused to let them stay and were taking them back to Syria when they were killed.