On Friday, Defense Secretary Ash Carter called out Russia for bombing a Syrian rebel group that's backed by the U.S.
Since last year, American and Russian warplanes have shared the skies over Syria while supporting different sides in the civil war. Moscow backs the Assad dictatorship; the U.S. is arming rebels who've been trying to overthrow it.
The attack by Russian fighter bombers on American-backed opposition forces appeared to be deliberate and to ignore repeated U.S. warnings.
It happened in southern Syria where two Su-34s bombed opposition forces, some of them trained and equipped by the Pentagon as part of the war against ISIS.
When the American-run command center which tracks Russian flights realized what had happened, it called the Russian command center in Syria -- on a hotline set up specifically to avoid incidents like this -- demanding the planes cease fire.
Two American F-18 jet fighters were dispatched to provide air cover for the troops on the ground as they tried to evacuate their casualties. By the time the F-18s arrived, the Russian planes were headed away, but were still close enough to see.
But when the F-18s broke away to refuel, the Russians returned for a second bombing run. Another call went out to the Russian command center in Syria, demanding that the planes wave off.
The crew of an airborne command post tried to contact the Russian pilots directly but got no response. The Su-34s conducted another bombing run, leaving a small number of opposition fighters dead on the ground.
It's been nearly nine months since Russia intervened in Syria's civil war, and although Russian planes have flown close to American aircraft before, this was the most serious incident between the two militaries.
Aides to Defense Secretary Carter described him as irate at the Russian action, although publicly he confined himself to calling them unprofessional.