The closure of the Rabia Crossing between Iraq and Syria earlier this month was part of a growing trend of Syrian border crossings falling under rebel control. While in Turkey the crossings have given rebels direct access to foreign supplies, Iraq has so far chosen to just close those crossings, it takes supply routes away from the Syrian government.
This has left Syria with fewer and fewer land routes to the outside world, and the al-Walid-at Tanf crossing is the last ground route into neighboring Iraq that government officials can use, along a roughly 600 km border.
Its a problem for Syria, but maybe a worse one for Iraq, as the rebels taking those border areas are with the al-Nusra Front, which is closely aligned with al-Qaeda in Iraq (AQI). AQI recently took credit for assassinating 48 Syrian soldiers who had fled into Iraq. The Syrians fled the loss of the Rabia crossing.
With such a broad swath of border under de facto al-Nusra control, closing the official crossings is likely to have a limited impact on the flow of fighters back and forth, and could mean an easy influx of Syrian rebel fighters into Sunni western Iraq, where AQI has been trying to use long-standing political protests as a jumping off point for a new rebellion.