USA troops completed a withdrawal from Iraq in 2011, eight years after they toppled Saddam Hussein's rule, only to return in much smaller numbers in 2014 to fight IS, which in 2014 seized large swathes of territory in both Iraq and Syria.
Speaking at Stanford University after being introduced by former top diplomat Condoleezza Rice, Tillerson said the Trump administration was determined not to repeat President Barack Obama's "mistake" when he withdrew USA troops from Iraq in 2011.
"It is crucial for our national interest to maintain a military and diplomatic presence in Syria", Tillerson said in a speech on USA policy in this conflict at Stanford University, California.
Secretary of State Rex Tillerson said Wednesday it was "crucial" for the United States to maintain an open-ended military and diplomatic presence in Syria to prevent Islamic State militants from regrouping, to thwart Iranian influence and ultimately to see the exit of President Bashar Assad.
Billed as the Trump administration's new strategy on Syria, the announcement will prolong the risks and redefine the mission for the USA military, which has for years sought to define its operations in Syria along more narrow lines of battling Islamic State and has about 2,000 US ground forces in the country.
"We ask all stakeholders in Syria's future to do the same".
Secretary of State Rex Tillerson, in a speech at Stanford University, made clear the United States would work diplomatically toward Assad's exit from power, but called for " patience" - an acknowledgment that Assad has been bolstered by Russian Federation and Iran and is unlikely to leave power immediately. "We understand that some Americans are sceptical of continued involvement in Syria", Tillerson said, but he argued such engagement was necessary to assure the IS "does not present a threat to the homeland and do not resurface in a new form". "ISIS presently has one foot in the grave and by maintaining an American military presence in Syria until the full and complete defeat of ISIS is achieved, it will soon have two".
Backed by Russian Federation and Iran, Assad has reasserted control over much of Syria.
Tillerson's urging of patience was the clearest indication yet of Washington's acknowledgment that Assad's stronger position in Syria, bolstered by Russian Federation and Iran, meant he would not leave power immediately.
Earlier in the day, Tillerson had said the USA would continue to work with Turkey and consider Ankara's concerns about the PKK.
The plans of the coalition have fueled Ankara's resentment over U.S. arming and supporting Kurdish forces in Syria and prompted Turkey to summon a United States top diplomat. The U.N. mediation has languished for years without any progress and fighting between Assad's military and rebel groups persists.
Tillerson said there are also other reasons for the United States to remain engaged in Syria. The NATO ally is fiercely opposed to an expanded training program for Kurdish and Arab border guards in Syria.