There are 4 winners in the US withdrawal from Syria - and they're all US adversaries

While the primary mission of the US military in Syria has been the "enduring defeat of ISIS," it has also been to counter troubling Iranian activities in the region.

Secretary of State Mike Pompeo said in October that a US objective was "the removal of all Iranian and Iranian-backed forces from Syria." One month earlier, White House National Security Adviser John Bolton stressed that US troops would not "leave as long as Iranian troops are outside Iranian borders and that includes Iranian proxies and militias."

The US military presence in Iran has disrupted Iranian activities, but withdrawing US troops from Syria will permit Iran to more easily pursue its interests.

The US has repeatedly expressed dissatisfaction with Syrian President Bashar al-Assad, with some observers assessing that he might eventually be removed from power. The US military placed at least limited restraints on the Syrian military as well.

Pulling US troops out of Syria risks emboldening Assad as he feels more secure and his enemies, which have fought alongside American forces, are left more vulnerable. With the threat posed by ISIS degraded, Assad could, with the support of his Russian and Iranian allies, focus his efforts on retaking territory lost in the civil war.

And, as America will no longer have a military presence in country, the Syrian regime, in coordination with Moscow and Tehran, will be free to dictate the post-war outcome.

While President Donald Trump insists that ISIS is defeated, the latest estimates suggest that there could be as many as 30,000 ISIS fighters in Iraq and Syria. Secretary of Defense James Mattis has argued that the US needs to maintain its military presence in Syria to prevent the resurgence of ISIS.

"Getting rid of the caliphate doesn't mean you then blindly say okay, we got rid of it, march out, and then wonder why the caliphate comes back," Mattis said in September.