As Turkey continues its military offensive in Syria, America has a dilemma over whether to stand by the Kurds who helped fight IS.
Syria’s civil war is entering a new, potentially more dangerous phase.
Turkey is throwing its military might into the conflict, raising the stakes.
Turkey’s involvement is easily explained. For much of the war, the best fighters in the battle with Islamic State (IS) were Kurds.
America armed, trained and supported them and, in turn, they spearheaded the assault on IS.
But Kurds are the Turks’ old enemies, for decades a thorn in their southern flank where some want to create a new Kurdish homeland.
The Turks have looked on in horror as their foes have grown stronger under US patronage.
They say they have sought and never received assurances from the Americans about the Kurds and now have no choice but to neutralise their threat.
They accuse the Kurds of terrorist activity over their southern border.
So they have launched Operation Olive Branch using tanks, air power and artillery to attack the Kurds in the Afrin region of northern Syria.
Turkey is backed up by Syrian rebel fighters who were, until recently, on the same side as the Kurds they are now attacking.
To the east of Afrin is another far bigger swathe of Kurdish controlled territory where America has deployed its military forces in secret bases.
Last week US Secretary of State Rex Tillerson raised the possibility of a 30,000-strong “border force”, sending shivers through the Turkish government.
It saw the plan as a plot that would end up with tens of thousands of armed Kurds permanently stationed on its southern border.
Diplomatically it was not Mr Tillerson’s finest hour and helped precipitate the Turkish offensive.
America has a dilemma. It has a duty to its Nato ally Turkey and so far seems to have given the Turkish military the room to carry out this operation against the Kurds.
But the honourable choice, arguably, would be to stand by the Kurdish men and women whose courage did so much to destroy IS.
And remember, Kurds fought and died so that America did not have to send ground troops of its own.
If America deserts the Kurds, it will send a clear message to any other potential US ally ever considering doing America’s military bidding – we will use you when it suits us but throw you under the bus, or Turkish tanks, when you are no longer useful.