In what could prove to be the single biggest blunder of the entire US war in Syria, US warplanes today attacked a Syrian army base in the Jebel Tharda area on Deir Ezzor Province, in the ISIS-dominated eastern portion of the country, killing at least 83 Syrian soldiers and wounding 120 others.

Pentagon officials claimed they thought the site was “an ISIS tank position,” and admitted to not giving Russia a precise location before the attacks. Russian officials slammed the US over that, saying it underscored the Pentagon’s “stubborn refusal” to coordinate actions properly.

US warplanes killing 83 Syrian troops during a ceasefire may not be the worst of the story, incredibly enough. Those troops had been defending the area from ISIS, who quickly overran what was left of the base’s defenses, and are now even closer to the Deir Ezzor airport.

The airport has been one of the last major government holdouts in the Deir Ezzor capital, and at times the Syrian warplanes flying out of the airport were the only thing keeping ISIS from overrunning the entire eastern half of the country. The US airstrikes seriously softened up the defenses in the area, and might finally do what years of ISIS offensives couldn’t, put ISIS in control of the airport.

US officials expressed “regret” over the killing of the Syrian troops, but condemned Russia for requesting an emergency UN Security Council meeting, saying that the Russians “had blood on their hands” from the war themselves and wasn’t in a position to criticize the US botches.

According to Centcom, the Syrian troops were all “out in the open” at the base, and they also destroyed six military APCs and one tank. They reported they were “watching” the Syrian Army base for days before the attacks, figuring everyone there was ISIS.