Up to now the official US position has been that its forces would stay on the ground in Syria to secure the enduring defeat of the Islamic State (Isis) group.
"Nobody is declaring a mission accomplished", said Brett McGurk, the US special envoy to the global coalition to defeat Isis, only two weeks ago.
But over recent months an additional narrative has been intruding, certainly among the more strident foreign policy voices in the Trump administration. A longstanding presence in Syria, it was argued, would help to contain Iran and counter Russia's growing influence in the region.
US ground troops first became involved in Syria in Autumn 2015 when then President Barack Obama sent in a small number of special forces to train and advise local Kurdish fighters who were fighting Isis. The US did this reluctantly after several attempts at arming anti-Isis groups had descended into chaos.
Over the intervening years the numbers of US troops in Syria increased, standing today at some 2,000, though some estimates place the number perhaps even higher. A network of bases and airstrips has been established in an arc across the north-eastern part of the country.
But what is their strategic purpose now? Isis is well on the way to being defeated. Syria's President Assad remains in place due to the support of his allies in Moscow and Tehran. If the goal now is to contain Iran or Russia's rising influence in the region, then 2,000 troops strung out across a vast swathe of territory may be too small a force to do this.
President Donald Trump's decision is thus in this sense logical, and it fits in with his own apparent hostility - despite a lot of bellicose rhetoric - to entanglements in foreign wars.
Others might argue though that the presence of US troops gives the US "skin in the game". It is an important presence, and on occasion US forces have been involved in direct clashes with pro-Iranian militias and Russian military contractors who tried to attack the positions of their allies.
Isis may be largely defeated, but what is to happen in the roughly one third of Syria that remains outside the control of President Assad and his allies? Could a new phase of the civil war ensue? And if large parts of the country descend into renewed chaos, something related to Isis or similar could easily emerge again.
The US has also played an important role in bolstering Kurdish groups in northern Syria who have been the key local element in defeating Isis. But these groups are seen by Turkey as a significant threat. It is telling that the Trump policy shift has come at a time when Washington and Ankara are trying to navigate a new bout of tensions, with the Turkish authorities warning that they plan to strike further into Syria against the self-same Kurdish elements.
So has Washington done a deal with Ankara? What security guarantees will there be for Washington's local allies going forward? And if the Kurds are effectively abandoned to their fate, what does it say for the reliability of the US as an ally in future conflicts, should local fighters be encouraged to align themselves with Washington?
But above all there will now be renewed questions about the Trump administration's whole approach to the region. What are its strategic goals? What are America's enduring interests there? And what means need to be invested to secure these goals?
There is no doubt that the Middle East - once Washington's crucial energy supplier and a focus for superpower competition - is today less important in purely geo-strategic terms to the US than it once was. But it remains a region of great instability and one of huge continuing importance to Washington's closest European allies, who confront its many problems just across the Mediterranean.
So Washington needs a coherent policy, one that extends beyond a simple slogan of "containing Iran".
With President Trump it often appears that US policy is unduly aligned with Saudi Arabia and Israel - or more accurately the approach of two influential figures, de facto Saudi leader Mohammed bin Salman and Israeli PM Benjamin Netanyahu - who are perhaps taking advantage of Washington's perceived weakness in the region to promote their own particular policy views.
President Trump's decision reverses the much-rehearsed official lines of both the Pentagon and the state department, and it places Washington's Kurdish allies in greater jeopardy.
A US pullout will only revive questions about the Trump administration's whole approach to the region which, while perhaps less important in Washington's calculations, still has the potential for explosive disruption and conflict.