The artillery barrage was so intense that the American commandos dived into foxholes for protection, emerging covered in flying dirt and debris to fire back at a column of tanks advancing under the heavy shelling. It was the opening salvo in a nearly four-hour assault in February by around 500 pro-Syrian government forces — including Russian mercenaries — that threatened to inflame already-simmering tensions between Washington and Moscow.

In the end, 200 to 300 of the attacking fighters were killed. The others retreated under merciless airstrikes from the United States, returning later to retrieve their battlefield dead. None of the Americans at the small outpost in eastern Syria — about 40 by the end of the firefight — were harmed…

American military officials repeatedly warned about the growing mass of troops. But Russian military officials said they had no control over the fighters assembling near the river — even though American surveillance equipment monitoring radio transmissions had revealed the ground force was speaking in Russian.