A contingent of about 20 British special forces pulled out of the facility less than 24 hours before the U.S. tracked Russian aircraft on June 16 flying across Syria to the garrison, according to U.S. military and intelligence officials briefed on the strike. The aircraft dropped cluster munitions on the target, according to U.S. officials and rebel commanders.

After that first Russian strike, officers with the U.S. military’s Central Command air operations center in Qatar called their counterparts in Russia’s air campaign headquarters in Latakia, Syria, U.S. officials said. The American officers told the Russians that the garrison was part of the U.S. campaign against Islamic State and shouldn’t be attacked.

Roughly 90 minutes after the U.S. warning was delivered, U.S. aircraft circling nearby watched as the Russians launched a second wave of strikes against the garrison.