The beginning of 2017 is shaping up to be a critical period in the Syrian Kurds’ campaign to defeat Islamic State and secure a measure of political autonomy in their homeland. How President-elect Trump manages the competing interests and alliances in Syria will directly affect the Kurds’ destiny. He ought to stand with these steadfast American partners.

Since 2012 the Syrian Kurds have occupied a precarious political position. They have been crucial American allies against ISIS. The Russian government has also courted them,...