The US had ditched a plan to arm Syrian rebel factions over a year ago, but the start of Russian airstrikes against ISIS and other factions has the US going back to that strategy, and throwing large amounts of anti-tank missiles at the various rebel factions in the nation’s west.
The initial shift was because of America’s shift in focus from early Civil War goals of regime change to fighting against ISIS. The groups getting the tow missiles aren’t fighting ISIS, by and large, they’re fighting the government, and by extension the Russians.
The shipments seem aimed at turning the Syrian Civil War into a proxy war, and the fact that the US is overtly backing Free Syrian Army (FSA) factions that are fighting alongside al-Qaeda in Idlib and Hama, because that’s the focus of one of the first Russian-backed offensive.
So the US strategy in Syria is shifting once again, initially focused on regime change, then focused on destroying ISIS, and now, having given up more or less on both of those, they just want to see Russia fail, and the administration seems comfortable with the possibility that US-made weapons will be destroying Russian tanks and helicopters.
This proxy war is also bringing the US closer to a de facto alliance with al-Qaeda, as the efforts to undermine Russia run squarely through al-Qaeda-held Idlib, and “stopping” Russia at this point means further consolidating al-Qaeda control over northwest Syria.