In Syria (as elsewhere) the US has been trying to achieve a political goal by the use of proxy militias. It arms these players (countries or groups). It provides diplomatic cover for them. In the 80s they would have been presented as local ‘freedom fighters’. Now perhaps as fighters for ‘democracy’. The message is always about how these are local forces who are fighting a tyrant on behalf of their people. However; without US diplomatic and military support these proxies would not be able to fight. In Syria the US claims to be involved in the peace process but this is not credible. If the US had been interested in peace they could have promoted a national dialog from the start instead of fomenting a civil war by arming various factions. 
The US has tried to achieve its plan for Syria (which has nothing to do with what the Syrian people might want) by the use of proxies. However; the US is having trouble managing its proxies.
Turkey has been presented as a key ally in the ‘fight against terror’. Even when it shot down a Russian jet Turkey received full US support. But now matters are getting out of hand. Turkey has been shelling the PYD – Kurdish forces – in Syria . These forces have recently been armed by the US allegedly to fight ISIS.  The US has had to publicly ask its NATO ally to stop. 
Another problem is some of the groups supported by another US ally, Saudi Arabia. In Syria, Saudi Arabia supports a right-wing Islamist group Jaysh al-Islam.  The one-time leader of this group (he was killed in an airstrike in December 2015) initially made speeches of a virulently anti-Shia nature – calling for their expulsion from Damascus – i.e. he made calls for religious and ethnic cleansing. Later he toned down this rhetoric. The mostly likely explanation for this change is that he was advised to do so by his Saudi backers (and behind them his US backers).  This group is now part of the official opposition negotiating team backed by Saudi Arabia.
The problem for the US is that it is trying – once again – to gerrymander ‘democracy’ and acceptance of US dominated global capitalism onto a country where it just isn’t going to fit. They naively support various groups who no doubt tell their CIA handlers ‘Oh yes; we love democracy and freedom’, because they figure that that is what they have to say to get arms. Then once they get some strength they go off on their own; either they follow a sectarian ideology or they become simply totally corrupt or they run death squads or other unpalatable human rights practices. Â Or, like Turkey, they simply start pursuing their own regional agenda.
Countless thousands of totally innocent people have died, been maimed and/or have been rendered homeless by these naive strategies in Iraq, in Libya and in Syria. President Obama coolly brushes this fact away by explaining that the changes they are trying to bring about are a ‘generational challenge’. 
On Syria it remains to be seen if the US (with Russia’s help) can even at this late stage rein in what they have started, or if, perhaps, it is not already too late.