Tillerson said the experience of the US withdrawal from Iraq in 2011, which was followed by the rise of Isis and the US military's return to the region, necessitated an open-ended US presence in Syria to prevent a revival of Isis.
"We cannot repeat the mistake of 2011, where a premature departure from Iraq allowed al-Qaeda in Iraq to survive and eventually become Isis," Tillerson said.
But he also indicated that one of the biggest challenges of the post-Isis era is Iran's enhanced role. With Isis now beaten back into a small pocket of territory along the Iraq-Syria border, the US has to address the reality that Iran's support for Assad in Syria has given Tehran a vastly expanded reach, he said.
"Continued strategic threats to the US other than Isis persist. I am referring principally to Iran," he said. "Iran has dramatically strengthened its presence in Syria by deploying Iranian Revolutionary Guard troops; supporting Lebanese Hizbollah; and importing proxy forces from Iraq, Afghanistan, Pakistan and elsewhere. Through its position in Syria, Iran is in a stronger position to extend its track record of attacking US interests, allies and personnel in the region.
Squeezing Iran will, therefore, be one of the foremost goals of the continued US troop presence in Syria, he said, acknowledging that the project will be difficult.
"Syria remains a source of severe strategic problems and a major challenge for our diplomacy," Tillerson said. "But the United States will continue to remain engaged."
One of the starkest illustrations of the risks of the entanglement is unfolding now, as Turkey escalates threats to attack the Kurdish enclave of Afrin in northern Syria.
The area is controlled by Kurdish fighters from the People's Protection Units, or YPG, who are allied to the United States but did not directly participate in the fight against the Islamic State. They are closely tied to the Kurdistan Workers' Party, or PKK, which is waging war against Nato member and US ally Turkey.
The latest threat from Turkey was triggered by US military plans to train a 30,000-strong border force to protect the Kurdish-controlled area of northeastern Syria. Turkey regards such a force as a threat to its national security. Saying that the force would represent "an army of terrorists," Turkish President Recep Tayyip Erdogan has vowed to wage war on Syria's Kurds. Turkish tanks and troops have massed in the border region, and Erdogan has said an invasion could occur this week.
The United States would not feel compelled to defend the Afrin area because it did not feature in the war against Isis, according to statements by US officials in recent days.
"We don't consider them as part of our Defeat Isis operations, which is what we are doing there, and we do not support them" with training and advisory programmes, a Pentagon spokesman, Marine Corps Major Adrian Rankine-Galloway, told Turkey's state-owned Anadolu Agency, in comments he confirmed in an email.
"We are not involved with them at all," he said. "The groups that we support are exclusively involved in operations countering Daesh," he added, using an Arabic acronym for Isis.
The announcement of the border force, which has exposed contradictions between State Department and Pentagon policies in the region, has triggered one of the worst crises in years in the already fraught relationship between Turkey and the United States.
Turkish Foreign Minister Mevlut Cavusoglu told reporters after meeting Tillerson in Vancouver, Canada, that any resulting damage to Turkey's ties with the United States could be beyond repair.
"Such a development would damage Turkish-American ties in an irreversible manner," the Anadolu Agency quoted Cavusoglu as saying.
