Syria is ruled by a coalition of ill-repute, made up of Bashar Assad, the Iranian Quds Brigade, Hezbollah and Putin. The involvement of Russia has turned the Syrian war into a battle of international consequence. Syria is in the process of breaking up, and there is probably nothing anyone can do at this point to stop that from happening. The vast majority of Sunni Arabs, as well as the Syrian Kurds, will never submit to Damascus rule, so long as Bashar Assad is in power.
The United States, as the indispensable global power, still the sole world superpower in every respect, must take the lead in bringing the Syria war to an end. That means that the United States must support the creation of a safe-zone for refugees to remain inside Syrian territory or in a neighboring state. Turkey has nicely filled this role through its invasion, which will soon spread further south and encompass an area large enough for such a safe zone. But that is not enough. The United States must impose a no-fly zone over Syrian territory. It is costly, but can be done. The United States successfully established two no-fly zones over Iraq from the end of the first Gulf War through to the 2003 invasion. The Kurds in the north and the marsh Arabs in the south were protected. But that, too, is not enough. The Syrian Air Force must be grounded, and that can be accomplished by American sea-launch and air-launch cruise missiles strikes. At minimum, the Syrian Air Force runways can be cratered.
And the Syrian rebels must be armed with shoulder-held anti-tank and some anti-aircraft weapons. Perhaps Arab Special Forces from neighboring countries can manage and supervise these weapons deliveries.
Yes, this is complicated by the presence of the Russian military forces in Syria, but not to worry. Putin always stops, the moment he runs up against any immovable object. The actions proposed show appropriate firmness. They are not provocations.