So, what has changed?

The threat posed by the Islamic State

The area controlled by the Islamic State, also known as ISIS, has continued to shrink in recent months. The map above — last updated in mid-January — still shows two parts of Syria controlled by the militants, but almost all of it is essentially uninhabited. In reality, the ISIS territory has now shrunk to a tiny part of the village of Baghouz, which is expected to fall soon, as well.

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Despite the territorial advances, the past two months have shown that reclaiming the group’s territory doesn’t automatically result in victory over the group itself.

Thousands of Islamic State fighters are still believed to be in Syria and Iraq, and the U.S. decision to withdraw has raised concerns that this number may grow once again. So far, Kurdish fighters in Syria were able to rely on their ally, the United States, to hold their own territory (highlighted in green in the map above) and watch over hundreds of Western Islamic State detainees arrested as the group lost its territory.

Feeling abandoned by Washington, the Syrian Kurds are now threatening to release the detained ISIS fighters to focus on a potential battle for their own existence. That Kurdish response should not have come as a surprise, but neither Europe nor the United States appears to have a plan for what’s next.

Much to the dismay of U.S. allies, Trump subsequently urged Europe to take back its ISIS fighters — even though on Wednesday he personally intervened to stop the return to the United States of an Alabama woman who had joined the Islamic State. The reasons for the prevailing reluctance of the U.S. government and its European allies to take back their citizens are largely the same: They fear that courts may not be able to sentence returning fighters because of a lack of evidence, allowing former ISIS fighters to roam the streets of Paris, London or New York instead of being sent to prison.

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Since few nations are willing to repatriate their citizens, viable options out of that dilemma are limited, and the nations have opted to continue the approach they have pursued so far — with troops inside Syria.

Plans for a “safe zone” in northeastern Syria

The repercussions of Trump’s announcement of a full withdrawal in December also led to concerns that two U.S. allies might fight each other immediately after the American departure. The United States is both a formal ally of NATO member Turkey and the main backer of the Syrian Kurds. Turkey, meanwhile, considers the Syrian Kurds to be allied with Kurdish militants on its own soil and has vowed to attack them once U.S. forces have withdrawn.

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To prevent such an escalation, a “safe zone” between Turkey and territory held by the Syrian Kurds has been proposed. But a full U.S. withdrawal would make such a safe zone almost impossible, given that the two remaining powers that could patrol it — France and Britain — have warned Trump that they won’t stay behind if he follows through on his pullout.

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Concerns over growing influence of Iran

Other nations are more willing to fill the power vacuum this would create, with Iran positioned especially well as a key ally of Syrian President Bashar al-Assad. Syrian military forces have in recent years worked hand in hand both with Iranian-commanded militias and Russian troops. All three nations would be eager to expand their sway over the northeastern parts of the country, currently held by the U.S.-backed Kurdish fighters.

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When German Chancellor Angela Merkel discussed the U.S. troop withdrawal from Syria at the Munich Security Conference earlier this month, she expressed concerns also held by some U.S. officials: “Is it a good idea for the Americans to suddenly and quickly withdraw from Syria? Or will it once again strengthen the capacity of Iran and Russia to exert their influence there?”

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Thursday’s announcement to keep some troops in the country suggests that waiting to find out the answer to that question appears to be an increasingly risky bet for the Trump administration, too.

Louisa Loveluck contributed to this post.