The Americans had been furious about the convoy from the start. The Islamic State had cut a deal with the Syrian government and its Hezbollah militia allies to allow the fighters and their families safe passage to cross Syria from the Lebanese border in the west to ISIS-controlled territory in eastern Syria and Iraq.
Within days, American airstrikes cratered the highway in front of the convoy, stopping it in its tracks. American officials vowed not to let the convoy pass, or the fighters to return to the battlefield.
“Our coalition will help ensure that these terrorists can never enter Iraq or escape from what remains of their dwindling caliphate,” said Brett H. McGurk, the American presidential special envoy to the anti-ISIS coalition.
But that American line in the sand was wiped away with a telephone call last Friday from Russian military headquarters in Syria to American headquarters in Baghdad. Russia asked the United States to remove aerial reconnaissance over the convoy, which both sides knew would allow the convoy to proceed.
The request was part of what the military calls “deconfliction,” a process to make sure the Russian-backed Syrian forces and the American-backed Syrian Democratic Forces do not inadvertently attack one another while both are trying to battle ISIS.
Cooperation with the Russians was important since Russian-backed forces and American-supported forces were separately closing in on the Euphrates River city of Deir al-Zour, with both sides launching numerous air raids in the area.
Moreover, the convoy was pinned down near the town of Sukhna, well within the Russian side of the deconfliction line, in the area reserved for Russian warplanes to operate under a longstanding American-Russian agreement.
“The way the deconfliction has worked, there are certain areas where the Russians have a sway over what happens, and this is one of them,” said Mr. Joscelyn said.
The United States did not want to undercut a process it would need to rely on later.