Most Syrian rebel groups have as their first priority defeat of the Assad government. However, they will defend themselves against other more radical Islamist rebel groups if they are attacked which has happened with the Islamic State and Al Nusra front.
The US policy has been to emphasize first and foremost the battle against the Islamic State in Syria. The result is not only outright tension between US trained and supported groups but even attacks on them by other rebels. The most recently trained group of rebels was attacked and routed by the Al Nusra front. The US program to train and equip rebels to fight IS began in May. Early on in the U.S.-led bombing campaign against IS back in November of 2014 the Al-Qaeda-linked Al Nusra Front has been a target as well with disastrous results as far as US relationships with other rebel groups is concerned. The US has continued this policy even though the Al Nusra Front itself has fought against IS and is also a key force fighting Assad in areas such as Aleppo.The rebels trained by the US are now refusing to fight back against the Al Nusra front insisting they were told that they were to fight IS. Fighting against the Front not only exposes them to the type of devastating attack they have just recently suffered but will bring down the wrath of other rebel groups upon them. The US view must be that whoever pays the piper calls the tune.Back in May, Al-Nusra leader Abu Al-Golani said that the Front had no plans to target the West but said its options were open should the Americans continue attacks against them. Every group he noted has the right to defend themselves. He described the rebels supported by the US as "the arms" of the US government in Syria. The recent attack caused the US supported group not only to abandon their base but, as mentioned, to refuse to fight back. In recent months the Front defeated two other Western-back rebel groups forcing them to disband. The US push to form an effective proxy force on the ground to battle IS is in complete disarray.The US has recently upped its involvement in Syria by declaring that they will bomb any group whether Assad or rebel forces that attack US-backed rebels. This is unlikely to have any effect except to turn the vast majority or rebel forces even more against the US. The US first bombed Al-Nusra front last September causing outrage among almost all rebel groups. The Nusra Front has taken further steps to complicate US policy in Syria by withdrawing its forces fronting the Islamic State in areas that Turkey and the US plan to be a "safe zone" on the border with Turkey. The Front said the decision was taken to avoid cooperating with the US-Turkish plan to create a safe zone in northern Aleppo province along the Turkish border.