The US claims a 55-kilometer "exclusion" zone around its al-Tanf base in southern Syria where Syrian and Russian soldiers are not supposed to enter
While most of the Syrian War is centered at this point around the Idlib Province, the last major rebel bastion, there is also interest in mopping up some of the smaller rebel positions along the Jordan border, especially where ISIS and al-Qaeda are concerned.
This is a raising a dangerous risk of a confrontation between the US and Russia, because Russia and Syria are very keen to chase some of the Islamists off the border in the near future, and Russia has informed them some of these Islamists are relatively close to US military positions in the area.
The US has long been fine with Islamists on the Jordan border, and is warning Russia against taking any action against them, saying the US would view it as a “challenge” and that they would not hesitate to respond militarily if there is any move into the area.
This is all near the US base at al-Tanf, on the corner between Syria, Jordan, and Iraq. The US has long demanded everyone stay away from this base, except rebels of course. The Pentagon is insisting they have the “right of self-defense” to keep their troops in this part of Syria, despite Syria never having allowed them to enter in the first place, and the offensive being aimed at al-Qaeda forces nearby, not the US troops.
The US has long taken this bellicose posture regarding Tanf and other bases around Syria, and in the past has attacked Syrian troops and Russian military contractors for getting too close. With Russia’s military directly involved and giving the US advanced warning, clearly a US attack won’t be taken the same way as those past incidents.
This is likely to be a long-term source of tensions in southeastern Syria, as the US shows no sign of leaving Tanf, and has no real military goals for the troops there, beyond being there to pick fights with other troops that happen by.