US ground troops were attacked outside of the Syrian city of Manbij today, according to coalition spokesman Col. Ryan Dillon. They came under fire from Syrian rebels allied with Turkey, and US forces returned fire before fleeing back into Manbij.

This is a fight that many have feared was coming for months, and is particularly significant because the Syrian rebels attacking the US troops are the same rebel forces that the US was directly arming through CIA arms smuggling operations for years.

With the CIA program ended, the rebels are now aligned with Turkey, and Turkey has made clear since their invasion of Syria that they intended to try to capture Manbij militarily, to expel Kurdish forces. It is only the presence of US troops in Manbij that has deterred NATO member Turkey from attacking the city.

But Turkey’s been very publicly griping about the US support for the Kurds. That, combined with the Turkish-backed rebels claiming the US “abandoned” them by cutting aid, appears to have finally provoked some fighting.

This puts a final exclamation point on what a bad idea the CIA arms program turned out to be, as not only did it not accomplish the regime change goals of the time, it’s armed up rebel factions that are now eager to fight US troops. While Turkey clearly has an interest in preventing this going too far, it’s unlikely this is the last of the US clash with these rebels.