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.

Q. Are the Kurdish militants in Syria different from the Kurdish militants in Turkey?

A. That depends on whom you ask. The United States says they are different, arguing that Y.P.G. fighters are basically focused on protecting Kurdish areas in Syria from the ravages of the civil war and are successfully fighting the Islamic State militant group, which is based in eastern Syria. The Turks say the Y.P.G. fighters basically share the same goals as the P.K.K. — secession through armed struggle — and that the United States should regard them all as terrorists.

Q. Why has this difference of opinion between Turkey and the United States, which are NATO allies, escalated into a huge problem?

A. The United States is actively supporting the Y.P.G. fighters while Turkey is attacking them, which threatens to cause a deeper rupture in Turkish-American relations. Turkish forces have been shelling Y.P.G. positions in Syria since last weekend over American objections, and on Thursday Turkish officials intensified the pressure on the United States by accusing the Y.P.G. of responsibility for a bombing in Ankara on Wednesday that killed 28 people. President Recep Tayyip Erdogan of Turkey has said the United States must make a choice of loyalties, implying a further schism with the Americans is possible.

Q. How might the Kurdish dispute between Turkey and the United States affect the Syrian war?

A. It could further complicate and prolong the conflict, now five years old, between President Bashar al-Assad of Syria and his array of rebel foes. Turkey and the United States are among the most important opponents of Mr. Assad, whom they accuse of butchering his own people. So American-Turkish frictions over the Kurds can indirectly benefit him and his allies, Russia and Iran.

The Americans have strongly signaled that they will not join Turkish calls for a military ground operation in Syria, even as Mr. Assad’s forces, emboldened with Russian and Iranian help, are regaining territory and strengthening his position should peace talks take place. The United States also has made it clear that it will not be drawn into the possibility of a direct military confrontation with Russia in the Syrian conflict.