At least to 2006, ten years ago, thanks to the US Embassy cable published by Wikileaks:

https://wikileaks.org/plusd/cables/06DAMASCUS5399_a.html

Some coverage:

http://america.aljazeera.com/opinions/2015/10/wikileaks-cabl...

"The cables also show that U.S. support for efforts to overthrow the Syrian government beginning in 2011 were not a response to the Assad government’s repression of protests but rather a continuation of a years-long strategy by more directly violent means."

Without that cable, this would be impossible to prove. Helps understanding why Manning who leaked that was 9 months in solitary confinement, stripped naked:

https://www.theguardian.com/world/2011/mar/11/stripped-naked...