The basic answer is the attempted occupation and division of Turkey, began with the occupation of Constantinople and Smyrna (Izmir), the latter of which marks the start of the war, and formalized in the extremely punitive Treaty of Sevres.

This was regarded as totally unacceptable to the nationalist Turks like Mustafa Kemal (later Ataturk) who formed a national assembly, collecting the formerly-Ottoman army and leading it against the occupying forces until they were able to renegotiate the Treaty of Lausanne, which recognized Turkey as an independent nation within its current borders (with the exception of Hatay, which was later handed over by the French.)