Trump penned his response - in bold lettering from the kind of marker he favours - directly on a copy of the letter from Senator Lindsey Graham, R, and a bipartisan group of 11 other lawmakers who had written to the President on February 22.
The correspondence applauded the White House announcement on February 21 that "a small peacekeeping group of about 200" US troops will stay in Syria beyond the planned withdrawal of most American forces this spring.
The White House announcement of a residual force apparently misstated the total - military officials have described plans for a force of about 400 - and did not specify its mission or duration.
Graham and other lawmakers had nonetheless welcomed it as a sign that Trump had listened to concerns that a complete withdrawal would endanger fragile military gains against Isis.
"A stabilising force, which includes a small contingent of American troops and ground forces from our European allies, is essential to ensure stability and prevent the return of Isis," the senators wrote.
Graham said in an interview that he received the President's response today. The South Carolina senator said he took Trump's note to mean that he agreed with the lawmakers' argument that a small force would help preserve military gains in Syria.
"To the President's credit, he adjusted his policy," Graham said.
The letter-writers told Trump they had all attended the Munich Security Conference the previous weekend and heard strong support among allies in Europe and elsewhere for a continuing American presence.
Trump's response also included hand-drawn bracket marks around a paragraph that began: "Like you, we seek to ensure that all of the gains made in Syria are not lost, that Isis never returns, that Iran is not emboldened, and that we consolidate our gains" ahead of potential political negotiations.
On December 19, Trump had stunned advisers with a sudden announcement that the contingent of about 2000 US forces at the time would depart immediately.
"We have defeated Isis in Syria, my only reason for being there during the Trump Presidency," he wrote on Twitter.
Later that day the White House posted a video in which Trump declared, "They're all coming back and they're coming back now. We won."
Isis is all but defeated in Syria, and no longer holds significant territory. The main arguments for keeping a military presence in Syria, at least temporarily, are to counter any resurgence of the group, to protect Syrian Kurdish allies and to maintain leverage in the region as the eight-year Syrian war winds down.
There are now approximately 3000 US servicemembers in Syria, an increase of about a third from the force the US had long maintained against Isis. The additional troops are there to help execute a two-pronged mission: helping local partner forces finish off Isis holdouts in eastern Syria while also making preparations to remove most troops in coming months in keeping with Trump's wishes.
