The ISIS caliphate, the physical nation-state they tried to build in 2014–15, is largely in ruins. ISIS the terrorist organization still exists, and it still has thousands of fighters. It is still a threat, and an American retreat gives it the potential to re-create safe havens in Syrian territory.
Moreover, Trump’s retreat empowers both Iran and Russia — granting a great strategic gift to two of America’s chief geopolitical foes. When Vladimir Putin intervened in Syria’s civil war to save the Assad regime, Barack Obama famously warned that Russia was getting sucked into a “quagmire.” In fact, Russia’s intervention has so far been an unmitigated success. He helped tip the balance of power in the civil war, secured continued access to Russia’s naval base in Tartus, and restored Russian influence in the region to a level not seen since the Cold War.
As for Iran, it has propped up its Syrian ally, and American withdrawal will only strengthen its hand as it deploys its assets in close proximity to Israel, raising the possibility of broader conflict with America’s closest Middle Eastern ally.