The amazing speed with which the SAA has been taking back Daraa province has been due at least in part to the U.S. refusal to give any more support to 'rebel' groups in the region during the SAA's "Southern Offensive" -- although whether the U.S. decision was unilateral or the result of U.S.-Syria communications or U.S.-Russia talks, I don't know.
Regarding the larger picture of the U.S. involvement in Syria, from Elijah J. Magnier's July 13 report:
[...] According to the source involved in overseeing the entire military operation for the last years of war in Syria:
“President Assad was very clear in his answer to the US establishment. Syria (said Assad) is determined to liberate the entire Syrian territory regardless of the consequences. There is of course a price to be paid to obtain the liberation of north Syria, which is occupied by both the US and Turkey, neither of whom were invited by the Syrian government: this price is worth it”.
The American message is clear:”The US will leave al-Tanf crossing and abandon northeast Syria in al-Hasaka and Deir Ezzor as soon as possible. The only condition is for Russia and Syria to guarantee a total withdrawal of all Iranian forces from the Levant. The US is ready to leave the Kurds and let these continue their negotiation with Damascus. The US establishment will recognise Assad’s authority over Syria but Iran must leave”.
Assad responded: “Iranian forces and their allies came to Syria under an official request by the central government and will leave when this government asks the allied forces to leave, and only when all terrorists have been eradicated from the Levant”.
“You (said Assad) came to Syria without any permission and occupied our territory. It is therefore our duty to push you out by all means. You shall not obtain by negotiation and peace what you failed to obtain after seven years of war”.I sense a little puffery on the Syrian side of the communiques but to continue:
Russia played the role of postman for the exchange of the US-Assad messages. President Assad, however, informed the Americans that Iran is not interested in remaining in Syria once all terrorist Takfiris are killed and when its function is no longer required.
Moreover, Iran and Hezbollah consider their withdrawal both a fact and a necessity once Assad is no longer in need of their contribution.
However, there is still al-Qaeda in the Levant, and other jihadists in the north under Turkish control. Also, there is still ISIS in the northeast within the US-controlled area. All these can only be eliminated once the Syrian Army and its allies wage war against them.
[...]That's a big "however," but I hope the U.S. military command now sees the wisdom of not obstructing the Syrian army and its militias, including the Lebanese and Iranian ones.
No one wants to leave off fighting more than Hezbollah, which has lost so many fighters in Syria they stopped publishing KIA stats years ago. And Iran's military, which has lost not only many grunts but top commanders in Syria, is anxious to stop throwing mountains of money into the Syrian War.
As to 'bleeding' Iran by stringing out the war in Syria -- only people who don't understand the present dynamics in the Middle East or ignore them would push such plan -- and I hope the American command is coming around to seeing this, now that they've realized Islamic State wasn't defeated in Syria or Iraq. IS in Syria hightailed to desert but Iraq is far worse. IS lodged in Iraq's countryside and gaining ground as I write these words.
Which is to say the U.S. is going to have need for Iranian militias in Iraq.
That's not the half the considerations but the point is that it's self-defeating for the U.S. to use Syria as a staging area to fight Iran.