Alongside defeating IS and al-Qaida, Tillerson cited several longshot propositions as American goals: Securing a successful United Nations peace process, getting Syrian President Bashar Assad to leave power, ridding Syria of Iran's influence and eradicating all weapons of mass destruction in the country.
The continued presence of the IS in Syria despite the loss of a majority of its territory it is necessary, Tillerson said, to avoid a repeat of the security vacuum that followed the United States withdrawal from Iraq in 2011 and the conditions that allowed the IS and other terrorist groups to flourish in the region.
Tillerson, in a speech at Stanford University, called for "patience" on Assad's departure, the clearest indication yet of an acknowledgment that the Syrian president has managed to hold on to power with the help of Russian Federation and Iran as his forces battle anti-government insurgents funded by Western governments and rich Arab Gulf states.
"A stable, unified, and independent Syria ultimately requires post-Assad leadership in order to be successful". He called President Assad a brutal murderer of his own people who can never provide long-term stability.
The United States has deployed around 2,000 ground troops to Syria and its warplanes patrol over the east of the country, hunting remnants of IS, which was largely routed from the country previous year.
Washington is now helping the militia create a 30,000-strong force to secure Syria's borders with Iraq and Turkey and prevent the militants from returning, in a move opposed by Iran, Russia, Turkey, and Syria.
"As we have seen from Iran's proxy wars and public announcements, Iran seeks dominance in the Middle East and the destruction of our ally Israel", he said.More news: Evacuees increases as Mayon nears eruption
The training would focus on existing elements of the Syrian Democratic Forces (SDC), the US' main ally in the fight against the Islamic State (IS) in Syria.
In deciding to keep troops in Syria, Tillerson insisted that Washington is not being dragged into the almost seven-year-long Syrian civil war as a combatant.
Tillerson said his country called on all nations to exercise discipline in economically pressuring Assad and rebuilding Syria after a political transition.
Assad's departure "will create the conditions for a durable peace within Syria and security along the borders for Syria's neighbors", Tillerson added later during a question-and-answer session.
During his presidential campaign Donald Trump harshly criticized his predecessors over interventions and wars in Afghanistan, Iraq and Syria, saying it was costing the US unneeded military casualties and a waste of money.
DoD also acknowledged security concerns from North Atlantic Treaty Organisation ally Turkey and assured the USA will be transparent about ongoing counterterrorism efforts in Syria.
The United States must remain both diplomatically and militarily engaged in Syria to protect its own national security interests, Secretary of State Rex Tillerson said Wednesday. Critics say the US pullout added to Iraq's instability and allowed a local Al Qaeda affiliate morph into Islamic State. "But that change will come", he said.