(CNN)A large-scale attack by over 500 Syrian troops aligned with Bashar al-Assad on a base used by US troops and local US allies underscored the challenges faced by the Trump administration as it seeks to implement its long-term strategy for Syria, with both the Russian-backed regime and NATO ally Turkey recently conducting or threating attacks against America's primary ally in Syria.
After being shelled by pro-regime tanks, rockets and artillery, US troops were able to counterattack, with US artillery and aircraft killing some 100 pro-regime fighters and forcing them to retreat across the Euphrates River, two US military officials told CNN Wednesday.
However, the attack, which US officials assess to be aimed at capturing lucrative Syrian oil fields that US-backed forces captured from ISIS, underscores the diverse array of challenges the Trump administration faces as it seeks to ensure the long-term defeat of ISIS while simultaneously containing Iran's influence in the Middle East.
Secretary of State Rex Tillerson laid out the administration's goals in Syria during a speech at the Hoover Institution in California last month, saying the US wants to defeat ISIS and al Qaeda, help resolve the Syrian civil war, contain Iran's influence in Syria, help Syrian refugees return home and ensure that Syria is free of weapons of mass destruction.
Tillerson made clear that the US thinks the UN-led process aimed at ending the Syrian civil war will remove Assad from power.
"The United States believes that free and transparent elections, to include the participation of the Syrian diaspora who have been displaced -- all those who were forced to flee the conflict -- will result in the permanent departure of Assad and his family from power," Tillerson said.
But while ISIS has lost over 98% of its territory in Syria many of the other objectives outlined by Tillerson are still fraught with difficulty.
Difficult goals
In order to achieve its goals in Syria, the US has become reliant on its main ally in the fight against ISIS, the 50,000 strong Syrian Democratic Forces, a mix of Kurdish and Arab fighters who helped fight off Wednesday's attack by pro-regime forces.
The local US allies have proven to be an effective fighting force against ISIS and have also helped check Assad and Iran's presence in eastern Syria, with Russia and the US negotiating an agreed-upon "de-confliction" line to separate Russian-backed regime troops and US-backed forces.
US military officials have told CNN that the US has sent Special Operations Forces to partner with US-backed Syrian forces along the Euphrates River and Assad's regime has called the Syrian Democratic Forces traitors and has vowed to eject US troops from the country.
The Syrian Democratic Forces have managed to capture much of Syria's oil and gas infrastructure after ejecting ISIS from those areas. US officials believe that the regime, strapped for cash after a nearly eight-year-long civil war, hopes to recapture the oil fields and simultaneously deny the Syrian Democratic Forces a source of revenue and legitimacy.
The US-led coalition said in a statement that they assessed that Wednesday's attack by pro-regime forces was aimed at recapturing some of these oil fields.
To complicate matters, two US defense officials told CNN that the US military now assesses that Russian contractors, believed to be funded by the Russian government, participated in Wednesday's assault.
Asked about the attack, Pentagon spokesperson Dana White said that the US was "not looking for a conflict with the regime" but added that US forces "have the inherent right to defend ourselves."
Assad's regime has also ramped up its attacks against opposition forces, with US officials accusing his forces of using chemical weapons like chlorine in recent weeks.
The US has said it seeks to continue working with the Syrian Democratic Forces and other local groups to ensure that ISIS cannot return and to make sure that areas recaptured from the terror group are stabilized, as the US hopes the UN can broker a peaceful end to the civil war and bring about a transition away from Assad.
"We're just making certain that it's turned over responsibly to the locals and that the locals have a seat at the table in Geneva," Secretary of Defense James Mattis told reporters last month, referring to the UN-led process that is seeking a peaceful resolution to the Syrian civil war. Mattis has previously said that US forces would help support US diplomats working to resolve the conflict.
Tensions with Turkey
However, America's NATO ally Turkey says those same Kurdish groups are linked to Kurdish separatists who have fought a decade-long insurgency against the Turkish government.
Shortly after Tillerson's speech and an announcement by the US military that it intended to work with the Syrian Democratic Forces to establish a border security force in order to secure areas liberated from ISIS, Turkey launched a major military operation targeting Kurdish groups in Afrin, Syria.
While the US does not support groups in Afrin, they are linked to American allies elsewhere in Syria. Additionally, Turkey has threatened to launch a military operation against Manbij, Syria, a place where US troops do work with local Kurdish and Arab fighters. Turkey has demanded that US troops withdraw from that area.
The commander of the US-led coalition, Lt. Gen. Paul Funk, told reporters during a visit to Manbij Wednesday that the US had no plans to withdraw and would continue working with local forces that are opposed by Turkey.
Facing attacks by pro-regime forces and threats by a NATO ally, the approximately 2,000 US troops in Syria could find it increasingly difficult to help bring about the long list of objectives outlined by Tillerson last month without additional clashes that could widen the conflict.
