The Kurdish fighters have few options. They could turn to the government of Syrian President Bashar Assad and its allies Russia and Iran for support, but that would mean giving up the autonomy they built up in the north over nearly 30 percent of Syria. So far, the Kurds' feelers to Damascus have been fruitless.
Otherwise, the 60,000-fighter force, which was the core ground force in the US-backed campaign against Islamic State militants, may have to fight to its last soldier. In the war with IS, it lost 11,000 fighters.
The Turkish military operation, backed by warplanes and a formidable mobilization, has already targeted dozens of locations along a stretch of nearly 200 miles along the Syrian-Turkish border in its opening salvo. Turkish warplanes and artillery shells hit intelligence offices and security headquarters of the Kurdish militia, wheat silos and residential areas.
This is a look at what may lie ahead for the Kurds, and at what cost:
TURN TO ASSAD?
Hours before the Turkish attack, Syrian Kurdish officials said they reached out Damascus. It was a desperate public call at a time when the Kurds had their least leverage.
Officials in Damascus condemned the imminent attack. But their only message to the Kurds was for them to "come back to their senses."
The Kurds essentially shrugged off Assad's rule years ago at the height of Syria's civil war when the government had to pull its troops out of the mainly-Kurdish northeast to focus on rebels elsewhere. They set up their own administration with a level of self-rule unimaginable before — which they want to keep at least to some extent in any final deal with Assad.
Assad's government has firmly insisted it wants the Kurdish area back with no conditions.
Strategically, Turkey's military operation is beneficial for Damascus and Moscow, which like Ankara are loath to see the Kurdish autonomous entity in the area.
Moscow has always tried to draw the Kurdish group away from the United States. It has not overtly condemned Turkey's assault — since it has also long courted Ankara — but it has encouraged Damascus and the Kurdish group to talk.
When asked about calls for Moscow to mediate between the Syrian government and the Kurdish group, Russia's Foreign Minister Sergey Lavrov said: "We'll see what we can do."
Damascus is more likely to leverage the Turkish attack to regain control over the country's oil resources, most of them in Kurdish-controlled territory, according to Eurasia Group. So far, there is no sign Syrian troops are trying to take advantage of the offensive further north to reposition or breach Kurdish-held areas.
Damascus was inclined to "wait and see" on the Kurds, particularly after U.S. President Donald Trump first announced last year that he plans to withdraw from Syria, according to U.N. officials who have followed efforts for a final resolution in the civil war,
Now the Kurds have a far weaker hand.
"Dealing with the Syrian government and Russia may be the path to a resolution, but it will not be the way to put the brakes on (Turkey's) offensive," senior Kurdish official Aldar Khalil told The Associated Press. He called Trump's sudden pull-back decision "shameful."
The Kurds could also try more concessions to reach an arrangement with the U.S. that could appease Turkey.
Turkey has always sharply opposed Washington's strategy of fighting the Islamic State group by arming the Kurds. For years, Syria watchers said a confrontation between the Kurds and Turkey was inevitable without a political resolution.
Since August, Washington and Ankara were working on a "security mechanism" along the border that involved joint U.S.-Turkish patrols and a withdrawal of Kurdish fighters. Now it has floundered because Turkey dismissed it as all for show.
Bigger concessions could include pulling out cadres linked to the Kurdistan Workers' Party or the PKK — the insurgent group Turkey sees as the essential threat — removing heavy weaponry and maybe even including Turkish-backed Syrian opposition factions in running the local administration, said Dareen Khalifa of the International Crisis Group.
"A new U.S. offer to Turkey should include a bundle of military and political concessions from the (Syrian Kurdish group) in return for limiting the scope of the operation," Khalifa said. "These concessions would need to be matched with unwavering US. guarantees to limit the operation."
The prospects appear more out of reach with the unfolding assault.
Still, one senior Kurdish official still had hope the Pentagon may help turn things around. "It is a friendship over years of joint work. The Pentagon is a strong supporter," the official told AP, speaking on condition of anonymity to brief reporters.
FIGHT OR FLIGHT
In almost five years, the Kurdish-led fighters have upgraded from a guerrilla group to a more organized force working with and trained by the world's top military.
They are formidable, capable of waging urban battles and have vowed to defend the border zone, the heartland of Syria's Kurdish population.
But they are at a severe disadvantage with Turkish air superiority in a campaign Ankara has been preparing for more than a year.
An alternative is to keep their fighters further south where Turkey has no intention of going. That would preserve the bulk of their strength to fight another day.
Nick Heras, an expert of Syria's Kurds with the New American Security, said while U.S. troops have stepped aside from the north, Washington is still asking the Kurdish fighters to continue counter-IS operations further south.
But, he said, if it chooses that, it would an "option of last resort."