Three of the four Iraq-Syria ground routes
became unsuitable for ground resupply by fall
2012. First, historical Sunni militant activity
in Sinjar area suggests that this route would
not be useful for government forces due to
security concerns.
Second, Assad’s withdrawal
from Syria’s northeastern Kurdish areas left
this northern route unsuitable for a principal
overland support channel.Third, The Syrian
rebel seizure of the Al Qaim-Abu Kamal border
crossing point in early September 2012 closed
the Euphrates River belt to ground resupply
from Iraq.Source: http://understandingwar.org/sites/default/files/IranianStrategyinSyria-1MAY.pdf