Ultimately it comes down to how, just as much as why.
I've long advocated that America stop policing the Middle East, at least in terms of direct military operations. It has been catastrophic. Let the Russians waste their time there - in the meantime, use domestic oil and gas, while strangling the market from abusers such as Saudi Arabia, and shifting to green energy. It isn't like the Sauds have a real choice but to sell their oil while we ramp up domestic operations - you own them, through and through, and even a full withdrawal wouldn't stop the black gold. They'd be dead without American dollars.
That said, this was needlessly harsh, not to mention damaging. 70,000 ISIS fighters are likely to go free. This withdrawal took place not long after we convinced the Kurds to dismantle their defensive outposts. And, worst of all, it needn't have happened this way at all.
A smarter strategy would be to stop f**k with Assad, and arrange a ceasefire with the deal including how to safeguard the Kurds. Then make it clear to Turkey that any further incursion into Syrian soil will have serious consequences to your partnership.
They wouldn't care, because the Turks are America's enemy as well. Which is fine, but like we've discussed with China, it is time to draw a line. Turkey has been playing both sides. Let them know that they'll either f**k off, or they are out of NATO and the Russians can have them.
You can't really make deals with Iran anymore. That has been off the table for awhile. But you could at least given the Kurds time to prepare, as well as made an attempt to clean up the mess Obama made by opposing Assad in the first place.
I get the need to stop involving yourself, but the strategy could have been far less damaging.