Why aren't the Kurds entitled to self-determination?
Due to some Ottoman Empire provinces and colonial map-drawing?
The Kurds are US allies, secular Muslims, and were already doing fairly well at autonomy in Iraq and Eastern Syria. Carving out a new state would reduce Iran's influence (since we handed them control of Iraq) and diminish Assad's power. Would likely also pressure Turkey to resume peace negotiations with their Kurds.
But was a threat Trump could have brandished. Another would be kicking Turkey out of NATO. I like the first, not the latter threat. But Trump is good at unfulfilled threats and promises.