On 17 September 2016, the United States-led coalition in Syria launched an airstrike near the Deir ez-Zor Airport lasting from 3:55 p.m. to 4:56 p.m. local time, in which Syrian Arab Army (SAA) soldiers were killed conducting operations against the Islamic State. Russia reported that at least 62 SAA soldiers were killed, while the Syrian Observatory for Human Rights (SOHR) said at least 80 were killed and 120 wounded. The United States said that the intended target was Islamic State militants and that the attack on Syrian soldiers was due to a misidentification of ground forces while the Syrian and Russian governments claimed that it was an intentional attack against Syrian troops. The attack triggered "a diplomatic firestorm" with Russia calling an emergency United Nations Security Council meeting. Later, the Syrian government called off a ceasefire that had been the result of months of intense diplomatic efforts by the U.S. and Russian governments.
After the cancellation of the ceasefire by the Syrian government, an aid convoy near Aleppo was attacked, which the U.S. coalition's governments called a retaliatory attack by the Syrian and/or Russian governments, an accusation denied by both governments. The dispute led to an end of U.S.–Russian bilateral peace talks on Syria. Russia used the air raid to justify enhancing its missile air defenses in Syria and the U.S. Secretary of State John Kerry would go on to claim that the breakdown of the ceasefire helped lead to the eventual capture of rebel-held East Aleppo by Syrian government forces. The attack also led to Islamic State of Iraq and the Levant, which was besieging the Syrian-government held city of Deir ez-Zor, capturing multiple strategic points around the Deir ez-Zor airport.
Background Accusations of previous attacks
Previously, in December 2015, the Syrian government made an accusation, rejected by the U.S., that United States forces killed 3 Syrian soldiers and wounded 13 others during a Sunday evening attack on a camp in Deir al-Zour province. The U.S. led coalition had been targeting ISIL militants in Syria since September 2014 although it has never coordinated any attacks with the Syrian government, whose President the U.S. had been trying to overthrow.
U.S.–Russian brokered Syrian ceasefire
On Monday 12 September 2016 at 7 p.m. Syria time (1600 GMT), a U.S. and Russian brokered ceasefire, which was intended to last one week, had come into effect. The ceasefire had been the result of months of "intensive diplomacy," primarily between the U.S. Secretary of State John Kerry and the Russian Foreign Minister Sergey Lavrov, but also between U.S. President Barack Obama and Russian President Vladimir Putin, that had begun in July 2016. At the time of this attack, the ceasefire had been in place for more than its fifth day and was less than 48 hours away from its completion. Had its completion come to pass, it would have resulted in the U.S. and Russia jointly creating and running, per the ceasefire agreement, "the Joint Implementation Group" – an intelligence sharing group that would have been dedicated to coordinating U.S. and Russian attacks against al-Qaeda and ISIL in Syria. Some of the United States' top national security officials, particularly the Pentagon, were opposed to the creation of the Joint Implementation Group, fearing that its implementation would allow the Russian government to obtain important information about the functioning of the U.S. military. The agreement would have also seen the U.S. gain a veto, ensured by Russia, over the Syrian government's air force's combat missions over opposition and civilian-controlled areas, which at that time included the city of Aleppo.
Siege of Deir ez-Zor
Deir ez-Zor Governorate at the time had one of the few remaining Syrian Government strongholds in Eastern Syria. In May 2015, Islamic State of Iraq and the Levant (also referred to as IS, ISIS, ISIL, and Daesh) militants launched an offensive that captured Palmyra, most of Deir ez-Zor (the capital city of Deir ez-Zor Governorate) and its surrounding outskirts, and cut off the Syrian government's remaining ground supply line to Deir ez-Zor. Those parts of the city and its outskirts controlled by the Syrian government were then effectively under siege by ISIL, leaving supplies to be solely delivered by transport helicopters. Consequently, ISIL began launching frequent attacks against the Deir ez-Zor Airport with the aim of halting the delivery of supplies (these attacks would continue until the Syrian government fully secured the airbase on 10 September 2017). Government resupply was made possible by the Syrian government's control of the Thardah Mountains, a nearby artillery base, and other areas near the airport since control of these areas prevented ISIL from attacking resupply aircraft flying into and out of the airport. For this reason, Deir ez-Zor Airport and its surrounding areas were considered by the Syrian government to be strategically crucial to preventing ISIL from taking full control of Deir ez-Zor. The Thardah Mountains (Jabal Turdah in Arabic) refers to a group of hills and mountains west and south-west of the Deir ez-Zor Airbase, on some of which there were set up Syrian government defensive positions referred to as points. This range includes Thardah hill (point 1) in the south-east of the range, Kroum hill (tal Kroum) in the north of the range, a mountain (point 2) in between these last two hills that is also referred to as Thartah mountain, a hill (point 3) in the south-west of the range, and others. A pro-Syrian government news outlet reported that on 17 September 2016, prior to the airstrikes, the Syrian Army was in control of Kroum hill and points 1 and 2.
Events Aircraft involved The Russian Defence Ministry said the attack was conducted by four jets, including two A-10 (a single-seat aircraft operated solely by the U.S. military) and two F-16 close air support and ground attack aircraft and United States Central Command stated that the attack was carried out by American, British, Danish, and Australian warplanes. The Royal Danish Air Force released a statement saying that two of its F-16's were involved in the airstrikes and the British released a statement saying that their armed Reaper drones also took part in the operation. The Australian Defence Force released a statement that two Royal Australian Air Force (RAAF) F/A-18A were involved in the strike together with a RAAF E-7A Wedgetail AEW&C aircraft.
Incorrect information passed through the deconfliction hotline At the time, the U.S. central command in Qatar was in communication with Russia, an ally of the Syrian government, via a special U.S.–Russia deconfliction hotline. According to the U.S.'s investigation into this incident, this hotline was used for the first time on 17 September 2016 to inform the Russians about these impending airstrikes, which were planned to attack two target areas approximately 3 to 6 km southwest of the Deir Ezzor airfield. The U.S. coalition operator misinformed his Russian counterpart about the location of the impending strikes, telling the Russians that they were to occur 9 km south of the Deir Ezzor airfield when they were actually planned to occur 9 km south of Deir Ezzor city and approximately 3 to 6 km south of the airfield. This mistake, the coalition's report notes, "may have affected the Russian response to the notification and caused considerable confusion in the DT process."
The Coalition raid On 17 September 2016, the U.S.-led coalition began attacking Syrian troops at 3:55 p.m. Syria time (13:55Z). The bombs hit Syrian Army positions on the Tharda Mountain and at a nearby artillery base. Russia and Syria reported that the attack was carried out at 5 p.m. by the U.S.-led coalition, without naming the countries involved. The New York Times was reportedly informed by an anonymous CENTCOM official that "the strike began in the early evening, when planes attacked a group of vehicles that American surveillance aircraft had been watching for several days" after which U.S. military intelligence concluded that the group, which reportedly included at least one tank, belonged to ISIL. This same official then reportedly said that "the attack went on for about 20 minutes, with the planes destroying the vehicles and gunning down dozens of people in the open desert."
Russia called the U.S. Central Command (U.S. CENTCOM) in Qatar at 4:25 p.m. (14:25Z) but was unable to find the designated contact, who was, according to U.S. CENTCOM, away from his desk at the time. According to the U.S. coalition's investigation, the Russian operator "elected to wait to speak to their usual point of contact (POC) rather than pass the information immediately to the Battle Director" which "led to a delay of 27 minutes, during which 15 of the 37 strikes were conducted." Although according to the coalition report the Russian operator had elected to wait for his POC, there was also reportedly a subsequent Russian call to U.S. CENTCOM that was answered at 4:52 p.m. (14:52Z) and that resulted in the airstrikes being called off "within minutes," according to a U.S. CENTCOM spokesman, who also said that "a good amount of strikes" had already taken place. According to the U.S. coalition's investigation, at 4:56 p.m. (14:56Z) coalition aircraft ceased fire and left the area.
An anonymous senior officer from the 123rd Regiment of the Syrian Arab Army (whose unit was reportedly one of those targeted) told pro-government Al Masdar News that more than half of the Syrian Army soldiers killed by the airstrikes died at points 1 and 2, which are right next to (and to the west of) the Deir ez-Zor airport, and that "the majority of the airstrikes were not conducted near the front-lines" [of where ISIL and the Syrian Army were battling]. According to the pro-Syrian government outlet that conducted this interview, the airstrikes "took out" the Syrian Army's defensive positions at points 1 and 2, points that had formed their last lines of defense of the airport, causing the Syrian army units fighting at the outskirts of Jabal Thardeh to retreat to defend these points and that one week later, ISIL had forced the Syrian Army to completely withdraw from even these last two points.
Casualties The initial casualty estimate given by the Russian Syrian government was that 62 Syrian government soldiers were killed, while the Syrian Observatory for Human Rights (SOHR) reported the death toll to be over 80 with 120 wounded. The next day, the Syrian Observatory for Human Rights reported the death toll to be 90, with 30 wounded. In late September, Al Masdar News reported that the Syrian government said that 106 soldiers were killed at Jabal Thardeh by the Coalition, more than half of them in these strikes.
Subsequent ISIL attack on the mountain An unnamed military source claimed that an ISIL ground attack was conducted just seven minutes after the Coalition attack. Pro-Syrian government outlets later asserted that this claim provided evidence of U.S.–ISIL collaboration. The ISIL attack allowed them to take control of Tharda mountain, which was seen as strategic as it looks over the government-held Deir ez-Zor military airbase.
On 17 September, according to pro-government sources, the Syrian Army recaptured some points and by the end of the day, after heavy ground fighting combined with Russian and Syrian airstrikes that left over 38 ISIL fighters dead, the Syrian Army recaptured all of the positions they had lost on the mountain (including Tall Kroum). During the clashes, a Syrian MiG warplane was shot down by ISIL over the mountain and the pilot was killed. The next day, ISIL was once again in control of the mountain after the military withdrew, creating a serious threat to the airport's southern flank, according to Al-Masdar.