The recent surge of Nusra Front – a bitter and bloody rival of the Islamic State group despite their shared extremist ideology – has overrun strongholds in Syria’s Idlib province of two prominent rebel factions that proved unable to repel the assault despite getting arms and training from the U.S.

The opposition groups’ collapse marks a significant setback to Washington’s plan of partnering with more moderate brigades to fight the Islamic State group and other radicals.

The two primary targets of the Nusra Front’s attack are the Syria Revolutionaries Front and Harakat Hazm. While both rebel groups have received U.S. support, it never reached the levels that either deemed necessary to make significant advances against President Bashar Assad’s forces in Syria’s 3½-year-old civil war. At the same time, the link to the Americans also earned them the enmity of radical groups.

“The Nusra Front is deeply suspicious of both the SRF and Harakat Hazm because they receive support from the U.S.,” said Aron Lund, editor of Syria in Crisis, a website run by the Carnegie Endowment. “The U.S. is also quite open about training rebels to take on both the Islamic State and al-Qaida, to which the Nusra Front belongs. So from the Nusra Front’s perspective, these groups aren’t just troublesome rivals, they’re a pro-Western fifth column that are slowly being readied for a purge of jihadis.”