With the outbreak of WWI, the Ottoman Empire joins forces with the Central Powers of Germany and Austro-Hungary and fights mainly against the British Empire. To achieve their military objective, the Allies persuade the Arabs to rise up against the Ottomans. In return they are promised their own kingdom.
The Treaty of Sèvres sealed the fate of the Ottomans following WWI. But the victorious powers no longer feel bound to the agreements they made. The Middle East is carved up into British and French spheres of influence and the prospect of autonomy is held out to Kurds. The Ottoman Empire, however, is completely dismantled.
For the Kurds the dashing of their dreams for an independent state is the beginning of a conflict that radiates into the present. And even the ISIS terrorists invoke those events in its justifications.