Escalating tensions between Turkey and the United States, which now jeopardize their alliance in the Syria conflict, can be traced to the Kurds, a Middle East people who do not have a state of their own. Here are five questions about the Kurds and their role in the rapidly evolving events in Syria and Turkey:

Q. Who are the Kurds, where do they live and what do they want?

A. The Kurds are an indigenous ethnic group with a population of 25 million to 35 million. They are basically spread through four countries — Turkey, Syria, Iraq and Iran, with lesser numbers in Armenia and Azerbaijan. They primarily follow the Sunni branch of Islam, share cultural identities and speak variants of the Kurdish language. Historically they resided in the Zagros Mountains, a range that straddles parts of these countries, commonly known as Kurdistan or land of the Kurds. But they are divided politically, reflecting a long history of uprisings for autonomy that have repeatedly been crushed.

Q. Why have the Kurds become such a priority for Turkey?

A. Turkey has historically worried about aspirations of Kurdish autonomy because it has more Kurds — at least 15 million — than any other country. They reside mostly in the southeast, which shares borders with Syria, Iraq and Iran. The Turks have been suppressing a violent Kurdish insurgency since the late 1970s led by the Kurdish Workers’ Party, or P.K.K., a militant group regarded by both Turkey and the United States as a terrorist organization. A short-lived peace process collapsed last year.

Now the Turks say the P.K.K. is collaborating with Kurdish militants in northern Syria, known as the People’s Protection Units, or the Y.P.G., to establish an autonomous region spanning both countries.