Turkey is facing widespread condemnation for its military operation against Kurdish-led forces in northeastern Syria.
America, as the planet's most powerful nation, can offer crucial help.
President Donald Trump called for Arab partners to contribute more to the fight against ISIS in Syria, but Saudi backing could further complicate the war.
Russia, Iran and Syria suspect that the U.S. is trying to split Syria and the region as a whole in order to take advantage of the unrest.
Turkey and its insurgent allies are sweeping through Kurdish territory and risking an international conflict with the U.S., France, Syria and Iraq.
The U.S. and Russia have both contributed to the fight against ISIS, but their goals for postwar Syria differ.
After defeating ISIS, Iraq and Syria have called for a complete withdrawal of Turkish forces battling Kurds in both countries.
The U.S., Iran and Syria all have complicated relationships with Turkey, which sees the Syrian civil war as an opportunity to tackle Kurdish groups.
With fewer local allies, the U.S. may no longer be able to fight ISIS on the ground in Iraq or Syria.
The shifting dynamics of Syria's seven-year war have pitted the U.S. against NATO ally Turkey as well as Russia and Iran.
The Kurdish majority of the Pentagon-created Syrian Democratic Forces are leaving the U.S. battle against ISIS to fight a Turkish invasion.
A former West Point cadet who left the U.S. to join Kurdish militia groups fighting Syrian ISIS forces was interrogated by police at a Chicago hospital.
Russian Deputy Foreign Minister Sergey Ryabkov said U.S. military action against the Syrian military would be "unlawful."
The axis of Russia, Iran and Syria faced attacks from Turkey in Afrin and Western pressure to cease bombing rebels in eastern Ghouta.
Former Defense Secretary Chuck Hagel said the U.S. military "can't fix the problems in Afghanistan" and Russia "really holds the cards" in Syria.
An account affiliated with Turkey's so-called Operation Olive Branch claimed Kurdish fighters attacked the Free Syrian Army with chlorine gas.
As Kurdish forces attempt to defend their land from Turkey, Russia began the Sochi peace conference and its allies continued to gain ground.
As a Turkey-backed offensive neared Manbij, Syria, the head of U.S. Central Command said withdrawal was "not something we are looking into."
Neither Russian, Syrian nor U.S. forces have moved in to stop the Turkish invasion aimed at ousting Pentagon-backed Kurds from the enclave of Afrin.
After ISIS was mostly defeated in the east, the Syrian military set its sights on rebel-held Idlib, while Turkey targeted U.S.-backed Kurds in Afrin, Aleppo.
"The U.S. must step back from this grave mistake and not allow itself to be blackmailed by the terrorist outfit," Turkey's top diplomat said.
Trump "has done more to isolate North Korea & defeat ISIS in 8 months than Obama did in 8 years," the president's press secretary tweeted.