Russia's interests and Iran's are strong in realpolitik terms. Syria is Russia's only foothold in the Mediterranean Middle East. Iran regards Syria both as a buffer against Sunni ambitions in the region and as a land route between Iran and its proxy army in Lebanon — Hezbollah.
Neither Russia nor Iran will agree to dislodging Assad unless there is some practical guarantee that Syria's future includes the accommodation of these interests. And I haven't seen anything out of the White House or the Hill GOP responsibly addressing this reality.
Moreover, the Sunnis of the region, led by Saudi Arabia and other regional players, regard Syria as their business, given the Sunni majority within Syria. And they, too, see the civil war in Syria as an important chapter in the long project to draw new, post-colonial Middle East borders in their favor.
And there are the Kurds, a proud but stateless people spread across many countries, who regard what's happening in Iraq and Syria as their best chance to carve out a territorial homeland for themselves.
Trump presumably has very little clue about most of this, and I'm not confident that the Hill GOP leadership are much better informed, preoccupied as they are by their quest for the Holy Grail of tax breaks for the one percent.