The decision, which Trump had telegraphed with recent statements saying it was time for the United States to "get out" of Syria, appeared to conflict with senior military officials who have repeatedly outlined the need for an ongoing military presence in Syria.
Those officials have stressed the importance of preventing both an Isis resurgence and increased territorial gains by Russian and Iranian-backed Syrian forces, and of giving the United States leverage in upcoming political talks to resolve Syria's civil war.
In some ways, Trump has split the difference between his own desire for a quick exit, and military concerns about leaving a vacuum in Syria. By ordering a "conditions-based" departure, pegged to Isis destruction, but not giving a date, he has left wiggle-room for further discussion as to what that "destruction" means.
In comments yesterday, he said the Syria mission was "close to 100 per cent" accomplished, and said that "I want to get out. I want to bring our troops back home." There are currently about 2000 US troops in Syria.
At the same time, General Joseph Votel, head of the US Central Command, agreed that "a lot of very good military progress has been made" but said "the hard part, I think, is in front of us."
Votel, speaking at the US Institute of Peace, described the military's counter-Isis mission as including "stabilising, consolidating gains" and "addressing long-terms issues of reconstruction" along with US diplomatic and non-military aid efforts.
Trump came to office promising to stay out of foreign wars, and calling for a US withdrawal from Afghanistan. At the same time, however, he has repeatedly criticised former President Barack Obama for setting a timetable for withdrawal from Iraq, and said that the 2011 departure of US troops from that country was ill-advised and left the door open for the rise of Isis.
Last August, as he announced a troop surge in Afghanistan, Trump said that "conditions on the ground, not arbitrary timetables, will guide our strategy from now on." The "consequences of a rapid exit," he said, "are both predictable and unacceptable...a hasty withdrawal would create a vacuum that terrorists - including Isis and al-Qaeda - would instantly fill."
Director of National Intelligence Dan Coats told reporters today that yesterday's decision followed an "all hands on deck" meeting at the White House. He said he could not release any information about the intelligence community's assessment regarding the likelihood of an Isis resurgence in Syria in the near future if US troops leave now.
As he has formulated his decision on Syria, Trump has approached regional leaders, particularly in the Gulf, about contributing more to the effort in Syria. Both Saudi Arabia and the United Arab Emirates were once active participants in the US-led coalition there, but their efforts have diminished as they have fought their own war against rebel fighters in Yemen.
In December, Trump came away from a phone conversation with Saudi King Salman believing that he had an agreement for a US$4 billion dollar Saudi contribution to Syria stabilisation, although the Saudis were said to have been privately taken aback by Trump's version of a solid offer.
Late last month, the Saudi and Emirati national security advisers travelled to Washington to discuss Syria with General H.R. McMaster, Trump's outgoing national security adviser, and Trump has spoken by telephone in recent days with Gulf leaders.
In a call with Salman, the White House said, "the President and the King discussed joint efforts to ensure the enduring defeat of Isis and counter Iranian efforts to exploit the Syrian conflict to pursue its destabilising regional ambitions."
Much of Trump's concern about an ongoing military presence appears to revolve around money. The United States, he said yesterday, had got "nothing out of US$7 trillion [spent] in the Middle East over the last 17 years," a calculation that apparently included the Afghanistan war against the Taliban in South Asia, where he last year approved a US troop increase.
The US$7 trillion dollar figure appears to refer to projections of expenditures in the Middle East and South Asia through 2050, if current deployment levels continue.
It also includes substantial costs tied to veterans' care and disability benefits, and war-related domestic and diplomatic security measures. Without those inclusions, experts estimate the costs of US military operations in Afghanistan and the Middle East, including the 2003 invasion and subsequent occupation of Iraq, at significantly less. A Congressional Research Service estimate put the cost at US$1.6 trillion through to the end of 2016.
Votel, with Defence Secretary Jim Mattis, has repeatedly said in recent months that US troops would be staying in Syria for the foreseeable future to guarantee stability and a political resolution to the civil war, which initially created space for Isis to advance.
According to Brett McGurk, the State Department's special envoy to the counter-Islamic State coalition in Iraq and Syria, the United States has spent about US$100 million so far on stabilisation of areas liberated from the militants, half of which was spent on mine removal. Other members of the coalition, he said, have spent about the same amount.
Last week, Trump froze an additional US US$200 million for the effort, which had been announced in January by Rex Tillerson, his since-ousted secretary of state.
McGurk, speaking with Votel at the US Institute of Peace, said that expenditure of the money is currently under review, "looking at where it can be spent most effectively." He said the freezing of the money "has not hampered what we're doing in the field" in places such as Raqqa and other liberated areas, where the goal is to de-mine, remove rubble and provide basic services such as water and electricity to allow residents who fled the fighting to return home.
