Earlier the Turkish president visited Russia and the Turkish foreign minister visited Tehran. Those are a lot of talks between big and important countries and players in the conflict over Syria. None of them, not even Barzani, is in the U.S. camp.

I assume that this outbreak of diplomacy, which bypassed Washington, led to concern that the U.S. might be left out of a resolution in Syria. It had to play a card of its own. That is the most likely explanation for the sudden clashes in Hasakah where the Syrian Kurdish YPG is suddenly determined to kick out the Syrian army garrison that protects the Arab population there. U.S. special forces are "advising" these Kurds.

The Syrian army garrison is cut off and the Kurds are well able to overwhelm it. They issued an ultimatum to the Syrian soldiers to either lay down arms or to die. The aim of this move is the creation of north-eastern block in Syria that is completely under Kurdish control (and sprinkled with new U.S. bases.)