I mentioned some time ago that we needed to decide what our objective in Syria is after defeating ISIL:
The Obama administration ignored the logical consequences of saying Assad had to step down by waging a parallel war as a de facto ally of Assad against the common enemy of ISIL that put off enforcing that declaration. The defeat of the ISIL caliphate has exposed the wide gap between the stated preference for Assad to leave and the focus of military action on ISIL only. So what do we do now?
The question could be avoided as long as ISIL still held ground in Syria. We had to defeat ISIL in Iraq, build up friendly forces in Syria, and then defeat ISIL in Syria. But after that, what?
America avoided facing that question while in the first three steps (and I suspect avoiding that question is why Iraq War 2.0 has taken so long). We've finally finished the first three steps.
America now has to face the fourth step and either finish the job by targeting Assad and defeating Russia, Iran, and Hezbollah in the process; or by pulling out and accepting the consequences of Assad's continued rule, Russian and Iranian bases in Syria, Hezbollah's role on the winning side, and the loss of reputation by watching the Syrian Kurds and Syrian rebels we once supported get crushed by Assad and his allies.
And I suspect avoiding the question is why the final defeat of the ISIL caliphate last month took so long after the effective defeat of ISIL over a year ago.
And FFS, no we aren't refusing to accept victory in Syria over ISIL. Like every victory, it is not the end of history but the entry ticket to the next problem or problems.
Now, post caliphate, we don't want Iran to entrench in Syria. We don't want Iran to have an overland supply route to Syria and Lebanon (where Iran's ally and creation Hezbollah is a power within that formal state). And we don't want to look like we are abandoning local allies in the SDF (mostly Kurds).
Those objectives are difficult as long as Assad is in power. But when we wanted Assad to step down, we did nothing effective to make that happen. And now that Assad seems secure, the fact that we don't have high interests in western Syria means we won't exert a larger amount of power to get rid of Assad to prevent those bad things.
Now we need to get "lucky" for that to happen. And if we get lucky until that is sorted out the chaos will deepen.
Will we stay in eastern Syria to prevent those bad things, hoping we don't suffer casualties to achieve them? But if we have a Beirut barracks bombing or Battle of Mogadishu that forces us to confront our unwillingness to suffer casualties for those objectives, we will retreat and look bad.
I'd like alternative to staying in Syria for preventing bad things. Could we strengthen Iraq to cut off the land route right there? Could we work with Russia and Israel to get Iran out of Syria and to get the Syrian Kurds a level of autonomy that protects them?
