The US is playing hardball with Assad Jr:

RUMSFELD SAID at the daily Pentagon briefing that coalition forces shut down the oil pipeline, but that he was not sure whether it was the only pipeline to Syria and couldn’t say whether oil is still flowing between Iraq and Syria.
There are allegations that Syria has been receiving up to 200,000 barrels of oil per day through the pipeline — in violation of U.N. sanctions against Iraq.
Oil traders and U.S. officials say that a complete cut-off of Iraqi oil to Syria, which is believed to meet about one-third of Syria’s oil needs, would send a strong message to Damascus. The country’s economy is in trouble, with gross domestic product growth barely keeping up with population growth.

Assad has to be scared shitless. He has a Turkey to the north, israel to the south and west, and Iraq to the east. Two democracies and a newly liberated country with a couple thousand confident troops in his neighborhood. I don't think he has the heart to challenge us to much.