Where do the Kurds fit into Syria's war?

The future of Kurdish-led areas of northern and eastern Syria has been thrown into doubt by President Donald Trump's decision to withdraw U.S. troops who have helped to secure the region. Amounting to about one quarter of Syria, the area is the largest chunk of territory still outside the control of President Bashar al-Assad, who is backed by Russia and Iran. Trump said on Wednesday the United States would withdraw slowly "over a period of time" and would protect the U.S.-backed Kurdish fighters as Washington withdraws troops, but without giving a timetable.