It has been a long time coming, but Turkey was never going to tolerate an independent Kurdish military on its border. In the face of of the United States helping the Syrian Kurdish People's Protection Units (YPG) establish a 30,000-strong anti-Islamic State "border security force" in northern Syria, Turkey has massed forces along the northern Syrian border, started shelling Kurdish-Syrian territory and, according to Turkish President Recep Tayyip Erdogan, is preparing to invade.

Who are the players here?

A little under 20% of Turkey’s population is Kurdish, concentrated in its east, and Turkey has long resisted -- sometimes brutally -- attempts at Kurdish independence. There is also a small ethnic Turk population in northern Syria, which Turkey has armed and supported against other groups, including Islamic State and the YPG.